Talk:Book - A Text-book of Embryology

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CHAPTER V.

THE FORMATION OF THE BODY- WALL, OF THE INTESTINAL CANAL, AND OF THE FETAL MEMBRANES.

The formation of the fetal membranes occurs coincidcDtally with the production of the external form of the body of the embryo. These changes mark the division of the hollow sphere or vesicle of which the germ consists up to this stage into two essentially diflFerent parts — namely, the embryonic body and the fetal appendages, the latter of which are destined for the nutrition and protection of the growing embryo. Although the several processes by which are produced the diflFerent parts of the embryo and its various appendages go on simultaneously, it is necessary, for the sake of clearness, to consider successively the development of each structure from its inception to its completion.

THE FORMATION OF THE BODY-WALL AND OF THE INTESTINAL CANAL OF THE EMBRYO.

In the stages of development thus far considered, the part of the ovuni that is to become the embryo — that is, the embryonic area — is represented by a localized thickening of the wall of the blastodermic vesicle, of the shape and relative size shown in Fig. 33, which presents a surface view of the germ. On each side of the embryonic axis, represented by the notochord, is the paraxial mass of mesoderm, which has undergone partial segmentation to form the somites ; on the distal side of the paraxial column, the mesoderm has split into the somatic or parietal, and the splanchnic or visceral lamellae, between which is the body-cavity or coelom. The cavity of the germ until the occurrence of the transformations about to be described is one undivided compartment which is bounded by splanchnopleure ; and a conspicuous fea 79

80 TEXT-BOOK OF EMBRYOLOGY.

ture of the changes under consideration is the division of this cavity into two by the folding-in of the splanchnopleure composing its walls. It will be well to consider first the formatior of the body-wall and the accessory structures of the chick, and then to take up the special mollifications that are presented by mammals generally and by the human ovum in particular. The first indication of the foldings that lead to the differentiation of the embryo from the fetal appendages is seen upon the surface of the germ at a very early stage. A surface view of the germ — in the case of the chick on the first day of incubation — shows, at what becomes the head-end of the embryonic area, a transverse crescentic groove, with its concavity looking backward (Fig. 41) ; a similar groove is seen at the opposite extremity of the area, and also one at each lateral margin. These marginal grooves are depressions in the somatopleure. The elevated outer edges of the grooves form folds of somatopleure, designated respectively the head-fold, the tail-fold, and the lateral folds of the amnion. As these marginal grooves increase in length they meet each other and now constitute one continuous furrow, which encircles the embryonic area ; its outer elevated edge is the aninionfsihL This furrow, which may be called an inverted fold composed of splanchnopleure and somatopleure, progressively deepens and at the same time its bottom is carried inward toward a point vertically under the central region of the cmbryimic area ; that is, a fold composed of somato})leure •■4 gfJanchnopleure grows from all parts of the periphery «f Ac embryonic area toward the point indicated above, a fMik which corresponds to the site of the future umbilicus. Bf At iDgiowth <rf the edges of the fold, the cavity of the

» moie and more constricted (Plate II., Figs.

Sy, vatil Cmdly, with the completion of the infolding,

^^^'•"•■"•Avifcd iaio two parts of unequal size ; the smaller

<# t)liii»-fi|Ma^iftdiagliiiMnci, or intestinal canal of the em Vaffyil^ wWi» di^ luf j u m the yolk-sac or umbilical vesicle.

which the gut-tract commute vttelliiic duct (Plate II.,

kfcf of the ingrowing fold

niBgninu lUusLrntiiit: tha fbrmatlon of Itau fl-tal membniiiGs {miHlllIed frum Rnule). "The ipapes miirt.il ■ lioclj-uaviiy ' In Flgun^s X and 4 nn; merely Itie etirs-embri'oiilo portions of llie body-cavlly. For more recent mncepUona »a in the romiatiuii «t tbOM'

THE FORMATION OF THE BODY-WALL. 81

thos outlines and forms the walls of the intestinal canal, the aomatopleuric layer, which acconiiianios it, constitutes the lateral and ventral body-walls of the enibrvo. During the progress of this infolding of the splanchnopleure and the somatopleure, the part of the latter nicn)l)rane that forms the outer wall of the groove becomes lifted up to constitute the anmion-fold (Plate II., Fig. 3) ; by the continued upward growth of this amnion-fold and the {simultaneous settling down of the embryo upon tlio yolk-sac, the margins of the fold come to lie above the cml)ryonic IkkIv, and, approacliing each other, they fuse over its ba(^k, in this manner enclosing it in a cavity. It is obvious that the fold just described is a double layer of somatopleure. After the union of its edges, the two layers become completely scjwinited, the inner one constituting the amnion, while the outer layer is the false amnion, or serosa, or chorion (Plate 11., Fi^s. 4-()).

Since the infolding of the sphmchnopleure begins at the periphery of the much elongatcMl eml>ryonic area, the resulting gut-tract has the form of a straight tube extending from the head-end to the tail-end of the embryo (Plate IWX When the caudal and the cephalic portions of the s|)]am'lm(H pleuric fold have advanced but a comparatively short distance, in consequence of which the coinnuinicatinn between the gut-tnict and the umbilical vesi<*le is still widely o]>on, as shown in Plate II., Fig. 5, there is a eiil-de-^iie or ]>oeket formed of splanchnopleure at the head-end (»!' tlut embryo and a similar one at its tail-end ; these reee<ses are respectively the foregut and the hindgut, the orifiees of whieli are designated the intestinal portals. At this particular sta^rts therefore, the cavity of x\w crut-tnirt i< ineompletely closed off from that of the uinbilieal vesicle.

It is evident that the gut-tract, beiuL^ a tiiluilar cavitv encloseleurc, is lined with entodernial cells; this simple strai<j:ht tube develops subsequently into the adult intestinal canal and its associated ^^andular apparatus.

It has alreadv been pointed out that the lavcT of somatopleure which is iohh'd under the embryonie area in coni]>anv with the splanchnopleure constitutes the lateral and the ven

82 TEXT-BOOK OF EMBRYOLOGY.

tml walU of the Ixxly of the embryo. The fold continues to 'jji\\"dnoM from each side and from each end, and its erJ^ri? t'jniiti together and fuse in the median line of the v*?fjtral hurface of the bo<Iy.* At one place, however, fusion of the mli^t^aof the fold dfies not occur; this r^ion correkf{x>tMlif U} the umbilicus and is often designated the dermal nsreL Here the {jart of the somatopleure that form^ the UmIv'^wiiII ih c;ontinuous with that part of this membrane whi^'h ^^>n»^tituUfH the amnion (Plate II., Fig. 6). By the iiifoldiiiir of the H^imat^>pleure the body-cavity or pleuro|X'rit/>fj<-sil K|KUM« \HH^mien divided into an intra-embryonic and an ^'Xtni'^frnbryonic [lortion, the two communicating for a liffie through the Hmail annular space that encircles the proximal t'tnl of the vitelline duct; this is represented in the a/'X'.'/miia living figures.

Uy iUii- Huiphf prrntesH of folding, associated with the iin<jiial growth of different parts, the leaf-like fundament i'diui^ilutU^l bv the embryoiiie arc«i is differentiated into the \p4nly of the I'lnbryo; the ventral jK)rtion of. this body now i'4nmti^tH of two tiibeh, one within the other, of which the hrrmlh'r, lK>iiiidi'd by the Hphiiiehnopleure, is the intestinal canal, and the Iarg<'r, I'lu-lowd by the somatopleure, is the bodjr-caritjr, th** wallK of which are tin? walls of the body of the embryo. In the doival region is a third tube, the medullary canal; iM'twecn it and the dorsal wall of the intestine is the notochord, on itwh side of wliieh an* the somites (Fig. 44). The further evolution of this biKJv and the differentiation of its various orgjuis and systems will be? described in subse^juent sections.

THE AMNION.

The amnion is a inembmnous fluid-filled sac, which surrounds the fetus of certain gmups of vertebrate animals during a part of their perio<l of (l<>velopment. In mati, it is

' Failuro of uni<m of tlio wimntoplciiric fohlM in the median line of the

thorax jjrrKluww the deformity known uh cleft Htennim ; while lack of fusion

of the hiteral halven of the alxloiniiial wall n^Mnlts in an extra-alxlominal

• INmition of the inteHtincH, or, if in lesKer dej^ree, in exstrophy %£ the bladder.

THE AMNION. 83

found as early as the fourteenth day,^ before the medullary groove has closed to form the neural canal ; it attains its maximum size by the end of the sixth month and persists until the end of gestation. It constitutes a loose envelope for the fetus, being attached to the abdominal wall of the latter at the margins of the umbilicus, and loosely enveloping the umbilical cord (see Plate III., Fig. 2).

An amnion is found in birds, reptiles, and mammals, these groups being classed together as Anmiota, while fishes and amphibians, which are without an amnion, constitute the class Anamnia.

The first indication of the growth of the amnion is apparent at a comparatively early stage of development. A surfaceview of the blastodermic vesicle of the first- day of incubation in the case of the chick shows a curved line or marking at the anterior edge of the embryonic area (Fig. 41) ; this is the anterior marginal groove, in front of which is another marking, the head-fold of the amnion. Very soon the lateral and posterior marginal grooves appear at the sides and posterior edge respectively of the embryonic area ; the outer elevated edges of these marginal grooves constitute the lateral folds and the tail-fold of the amnion. The grooves and folds increase in length in each direction until they meet, when they form one continuous furrow, which circumscribes the embryonic area, and the outer elevated edge of which is the amnion fold. The groove involves both the somatopleure and the splanchnopleure, constituting the inverted fold of these two structures that grows in to form the body- wall and the wall of the gut-tract, while the amnion fold is composed of somatopleure alone (Plate IT.). This separation of the somatopleure and the splanchnopleure enlarges the extraembryonic portion of the bofly-cavity. The amnion fold continues to grow upward, and finally its edges meet and fuse over the back of the -embryo, the line of union being the amniotic suture ; the suture closes first at the head-end of the embryo and last at the tail-end. After the union of the edges

' Ret*enlly it lias been found complete in an ovum estimated to be four (lavs old.

84 TEXT-BOOK OF EMBRYOLOGY.

of the fold, its inner layer, consisting of ectoderm and parietal mesoderm, separates from the outer layer to constitute the true amnion, whose enclosed s|>ace is the amniotic cavity ; the outer layer, which is merely a part of the general somatopleure, is the false amnion or serosa. It is apparent from this description that the amniotic cavity is lined with ectodermal epithelium and that its walls consist of somatopleure — that is, of ectoderm and parietal mesoilerm.

While the amnion fold is growing upward, the embryonic area — now undergoing ditferentiation into the embryonic body — is sinking down ujK)n the yolk-sac. The amnion fold does not grow uniformly in all parts of its periphery. The head-fold is produced first and constitutes a cap or hood covering the head of the embryo, which is forming simultaneously by the vcntnid growth of the somatopleure at the bottom of the marginal groove. It is only after the development of the head-fold is well a<lvanced that the lateral, and, later, the caudal, portions of the amnion-fold grow up to meet it. The head-fold is, for a time, destitute of mesodermic tissue, since it corresponds to that region of the wall of the blastodermic vesicle described on ])age 64 as the proamnion.

It htis lK»en shown (p. 54) that the amniotic cavity of mammals is produced not by the upgrowth of folds of somatt>pleure, but by a vacuolation of a portion of the cells of the inner ii»ll-mass (Fig. 28, p. 5")). Since the enveloping layer, which forms the roof or vault of the amniotic cavitv, constitutes the extra-embryonic cct<Mlerin (p. 05 1, this cavitv in mammals as in binls is lin<»d with ccttMhTmal cells, the floor t»t the cavitv being also ectodermal since it is i'ormed by the <Mni^r^'«mic disk, the amniotic* surface of which constitutes the •'mnrviioic eou>ilerm (Fig. 21>). Covering the ectodermal roof > A .uv^-r '^t nu*SiHlenn continuous with tlu* mesiHJerni of the Li'n- -iiic dLsk. The embryonic bud or <lisk, at first con"■' •«! rs> ^mwiitfii* <urf?HH\ the future dorsjil surface, becomes

• ia\vx^ Its eilgi*s curving towanl th<' o])jM»site

r -^HTTs. -=iriL'^- It should not be forgotten that the -5P-tt-:=. — -^^^ "iis ventral surface and at the ]MTipherv

Jeti'p surface of the enveloping

THE AMNION. 85

layer, and also that the embryonic ectoderm is likewise continuous at the periphery of the embryonic bud with that part of the enveloping layer which forms the vault of the amniotic cavity ; hence, after the ventral curvature of the embryonic bud, the periphery of which carries with it toward the ventral surface the amniotic ectoderm and mesoderm, we have practically the same conditions as obtain in the avian embryo as shown in Plate II., Figs. 4 and 5. While in the latter case the amnion is produced by the formation of folds, in the mammalian germ the same result is attained by the vacuolation of the inner cell-mass.

As the curving ventrally of the embryonic bud continues, the originally flat mass of cells composing it is converted into an imperfect tube, the lateral and ventral surfaces of which correspond with the former dorsal surface. This ventral folding of the embryonic bud produces the body of the embryo. As the folding includes the entodermal layer on the ventral surface of the embryonic bud, the blastodermic cavity is divided, as in the bird's germ, into the primitive intestinal canal and the yolk-sac (Fig. 48).

The amnion of man presents an important variation from that of all other Amniota, since the inner layer of the amnion-fold does not entirely sever its connection with the outer layer, but remains attached to it over the caudal pole of the embryo. In consequence of this attachment the true amnion is connected with the false amnion, and since the true amnion is continuous with the body- wall of the embryo, the caudal end of the embryonic body is attached to the false amnion or chorion bv a mass of mesodermic tissue called the allantoic stalk or belly-stalk, as seen in Fig. 47. The relation of the belly-stalk to the development of the allantois will be pointed out hereafter.

The space within the amnion — the amniotic cavity — is filled with the amniotic fluid or liquor amnii.

The amnion at first envelops only the sides and dorsum of the embryonic body, occupying the upper part of the cavity enclosed by the chorion, as shown in Plate II., Figs. 5 and

86 TEXT-BOOK OF EMBRYOLOGY.

6 ; the groove, or farrow, however, of which the amnion fold is the peripheral or outer elevated edge, becomes deeper, and the bottom of the groove is carried toward the middle of the future ventral surface of the embryo, its ventrad growth continuing until it reaches the position of the future umbilicus (Plate II. : Fig. 4, transverse section ; Figs. 5 and 6, longitudinal section). The layer of somatopleure constituting the inner wall of the groove — that is, on the side toward the embryonic area — becomes the lateral and ventral walls of the body of the embryo, as described above ; in this manner is effected the transition from the flattened or layer-like embryonic area to the definite form of the embryonic body. The ventral body-wall is continuous at the margins of the umbilicus witli the amnion, since the somatopleure, forming the outer boundary of the original groove, is a part of that membrane. After its completion, therefore, the amnion envelops tlic body of the embryo on every side, lying closely applied to it, since the amniotic cavity is at first very small. With the progress of development and the increase of the fluid the amnion requires more room, until, in the third month, it fills out the entire space within the chorion, with the inner surface of which it acquires a loose connection. The umbilical vesicle and the alhintois have meanwhile imdergone regression. The walls of the amniotic sjic contain contractile fibers; it is to these that the rhvthmical contractions observed in the amnion are due. Its lining is at first a single layer of flattened epithelial cells; at the fourth month the cells are cubical for the most part, but to some extent columnar.

The liquor amnii is a watery fluid having a s]>ecific gravity of 1.007, and containing about 1 per cent, of solids (albumin, urea, and grape sugar). The origin of the fluid is believed to be in the blood of the mother, the liquid portion of which transudes into the amniotic cjivity. The amniotic fluid increases in quantity until the sixth month of pregnancy ; from this time until the close of gestation it generally diminishes about one half. A pathological excess of the fluid constitutes the condition of hydramnios.

The Amction of the amniotic fluid is two-fold ; it serves as a buffer for the fetus, protecting it from mechanical violence, and it supplies the fetal tissues with water, since portions of it are from time to time swallowed. Evidence that the fetus swallows the fluid is aflTorded by direct observation of chicken embryos, and by the presence of epidermal cells, hairs, and fatty matter in the fetal alimentary canal. After the development of the bladder, the urine of the fetus is from time to time evacuated into the amniotic cavity.

The epidermis of the child in utero is protected against maceration in the amniotic fluid by the presence of a fatty coating, the vernix caseosa, which is a modified sebaceous secretion.

At the end of pregnancy, the amnion is loosely united with the chorion and the deciduae ; during birth it ruptures, and its fluid escapes.

THE YOLK-SAC.

The yolk-sac, or umbilical vesicle, as seen in the higher vertebrates, is a capacious sac attached by a narrow i>edicle, the vitelline duct, to the ventral surface of the embryonic intestinal canal, the duct passing through the umbilical aperture (Plate II., Fig. 6).

In order to appreciate more fully the function and the morphological relations of this structure, it is necessar}' to glance at the conditions that obtain in the several classes of vertebrate animals. In ova that develop outside of the body of the parent organism, a special dower of pabulum is provided for the nutrition of the embryo ; this dower is represented by the deutoplasm so abundant in telolecithal ova. In the case of amphibians, whose cleavage, it will be remembered, is holoblastic or total, the cells richest in deutoplasm are accumulated, after segmentation, in the floor of the archenteron ; this accumulation produces on the future ventral surface of the embryo a marked bulging, which constitutes the amphibian yolk-sac. As the embryo grows, it draws upon this store for its nutrition, in consequence of which the sac gradually shrinks, its cells being, for the most part.

liquefied nod absort)ed, while Bome of them cx>ntribute to the lining of the iiiteittiiial canal.

Ill a UghaT type, as exemplifled in sharks and dog-fiahGS, the yolk-sac is produced by a folding-in of the spIaDchnopleure and the soraatopleure, the walls of the sac being therefore constituted by both of these layers ; this folding-ia divides the arehenteroa into a smaller part, the intestinal canal, lying within the body of the embryo, and a larger cavity, the yolk-sac, situated outside of that body. The eplanchnopleuric layer of the yolk-sac is continuous with the wall of the intestuial canal, while its somatopleuric layer is continuous with the body-wall. A system of blood-vessels develops upon the yolk-sac, their function being to convey the nutritive material into the Ixidy of the embryo. These blood-vessels constitute the so-called Tascnlar area, which appears, in surface views, as a zone encircling the embryonic area, and, later, the embryo, since the latter reposes upon the proportionately much larger yolk-sac. As the contents of the sac becj)me absorbed, the latter shrinks, the splanchnopleurie layer slipping into the abdomen iif the embryo through the umbilical oi>ening, the somatopleuric layer contracting to close that aperture.

In the Asmiota (p. 83) the development and structure of the yolk-sac are modified by the presence of the amnion. In these groups the yolk-sac and the gut result from the division of the blastodermic cavity by the folding-in of the eplancli■oplcure alone, since the somatopleure grows away from the Bplanchnopleure to form the amnion-fold, and thus only partially invests the yolk-sac (Plate 11., Fig. 4),

Since the yolk-sac contains the store of food destined for the nutrition of embryos that develop outside of the maternal body, and since the mammalian embryo, which le.idsan intranterine existence, is endowed with a relatively small quantity of such store, the yolk-sac of mammals would seem to indicate the dcwent f)f the latter from oviparous ancestors. Further and strimger evidence of such descent is found in the fact that the eggs of the lowest order of mammals, the Monotrcniata, comprising the echidna and the ornithorhynchus, are " laid" and undergo triro-uterine development.

In the human embryo the umbilical vesicle is found partially constricted off from the intestinal canal by the end of the second week ; by the end of the third week the separation of the two cavities has advanced to such an extent that the vitelline duct is present, the sac attaining its maximum size by about the fourth week.

The function of the umbilical vesicle, as above intimated, is to serve as the organ of nutrition for the embryo during a certain period. The manner in which its blood-vessels develop will be considered in Chapter X. Their growth precedes that of the intra-embryonic portions of the vascular apparatus, the vascular area of the yolk-sac being the seat of the earliest blood-vessel formation. The vessels find their way into the body of the embryo along the vitelline duct, and consist of two vitelline arteries and two vitelline veins.

With the development of the allantois the yolk-sac retrogresses, the allantois succeeding it as the organ of nutrition and respiration. By the end of the sixth week the sac has shrunk to a narrow stalk, which is surrounded by the enlarged amnion, and which terminates in a knob ; at birth, the knob lies near the placenta (Plate V., Fig. 2), and the atrophic remnant of the stalk is one of the constituents of the umbilical cord.

THE ALLANTOIS.

The allantois is an embryonic structure which is found in those vertebrates possessing an amnion. Its growth is correlated with the retrogression of the umbilical vesicle, which structure it supplants as the organ of nutrition and respiration for the embryo.

Appearing at first as a little evagination or out-pocketing of the ventral wall of the gut-tract, the allantois finally becomes a pedunculated sac lying in the extra-embryonic part of the coelom (Plates II. and III.)> its stalk leaving the body-cavity proper through the umbilical opening. Being an outgrowth from the intestinal canal, the walls of the allantois are made up of splanchnopleure — that is, of entoderm and visceral mesoderm. Blood-vessels develop in the mesodermic stratum^ the principal trunks^ the two allantoic arteries and veins, being connected at their proximal ends with the primitive heart ; this system of vessels constitutes the allantoic circulation and is the avenue through which the growing eml)rj'o is supplied with nutritive material and oxygen. As the fundus of the allantois increases in size, it spreads itself out ujwn the inner surface of the false amnion or chorion (Plate III., Fig. 1), into whose villi its vascular tissue penetrates, and with which it becomes intimately blended. The union of the allantois and the false amnion produces the true chorion of some authors.

The human allantois presents a striking i)eculiarity as compared with that of birds and reptiles ; in man, the allantois

Fig. 47.— DIaprammatic sections roproRcntinp: growth nnd nrrnnpomcnt of the amnion in the earlioHt HtagcH of the human embryo (His).

develops not as a free sac projecting into the extra-embrj-onic bo<ly-cjivity, but as a mass of splanchnopleuric tissue which contains only a rudimentary cavity and which grows into the alxlominal stalk (Fig. 47 and Fig. 5S, hfd\^ Iwing guided bv that structure to the chorion. Moreover, while the human allantois is in effect an evagination of the ventral wall of the primitive gut-tract, its evagination begins before the gut-tract is const ricted-otf from the yolk-sac (Fig. 48).

THE ALLANTOIS, 91

The ftmctioii of the allantois in ogg-laying animals, and possibly in some others, is to serve as a nutritive and respiratory organ and as a receptacle for the fetal nrine : in man its cavity is exceedingly minute, and its chief function is to furnish a means of conveying blood-vessels from the embryo to the chorion.

Fig. 48.— Mesial section through an early human ovum (Oraf Spec) : a, AMominal Btalk ; b, amnion : r, yolk-.sac : d. hypoblaut ; c, me»obla!»t; /, vessels on wall of yolk-sac : g, primitive streak : h. allantois : i. medullary [»late ; j, early heart : k, mesoblast of chorion ; /, early villi ; m, chorionic mesoblast extending outward into villi.

The part of the allantois contained within the body of the embryo produces three structures of the adult organism : 1, the nrachus, an atrophic cord extending from the summit of the bladder to the umbilicus;^ 2, the urinary bladder; and 3, the first part of the urethra of the male, or the entire female urethra. The extra-embryonic portion shrinks after the appearance of the placenta and forms one of the constituents of the umbilical cord, its blood-vessels becoming the umbilical arteries and veins.

^ If the urachas remains patulous instead of becoming impervious, urine may escape at the umbilicus, and the condition is a variety o( urinary JUtukL

THE CHORION.

At the time when the false amnion is forming, the attenuated zona pellucida still surrounds the embryonic vesicle as the so-called prochorion, which unites with the false amnion, producing the primitive chorion. After the allantois has grown forth from the gut-tract and has spread itself over the inner surface of the primitive chorion, it becomes blended with the latter to constitute the tme chorion of some authors. The chorion, according to the above nomenclature, may be defined as the membrane which encloses the germ at the stage following the appearance of the amnion and the false amnion, and which has resulted from the fusion of the allantt)i8 with the primitive chorion ; or, ignoring the zona pellucida, the chorion results from the fusion of the allantois and the false amnion. Minot, however, defines the chorion as all that part of the extra-embryonic somatopleure which is not used in forming the true amnion, and hereafter in this work the wonl will be used in this sense. This definition limits the term to the outermost covering of the germ after the formation of the amnion (Plate III., Fig. 1).

The chorion consists of an outer ectodermic layer and

an inner lamella of mesodermic tissue. The mesoblastic layer

it thini being composed of from two to four layers of round,

OVtl|Or fusiform cells, and is at first devoid of blood-vessels.

The latter, in the form of capillaries, make their appearance

il KNne time during the second week, probably as extensions

^ the bUKxl-vessels of the allantois.

• t\m outer ectodermic or epiblastic cells of the chorion at

ft ttty tftrly period, certainly as early as the third day, pffoliferatioD to form a layer of tissue called the wbioh IB from one to several layers of cells in The tfophoblast layer is thickest at the place of eC dM ovum to the uterine mucosa. The inner ef il» iMfhebbH aie cttbical, and have large, finely

•vtl niicleL In the youngest human

of Peters, estimated to be three

was found to present many

qC etiiiids and buds, these

being the foundations^ of tlip future villi nf tlio cliorion. The tropliohlaet layer was not solid, but was honeycombed with little spacra or vacuoles filled with maternal blood, which spaces were partly lined with a nnclealed protoplasm, the early syncrtitun (Plate lA'., n). Even at this early stage, therefore, when the trophoblast strands or early villi are as yet devoid of a mesnblastic element, they are bathed with the maternal blood. Very soon the niesoblastic tissue of the chorion grows into the trophohlast strands, thus forming the permanent villus stems ; and during the seeond week capillaries extend into the stems, completing the foruiatioD of the fully developed villi of the chorion.

The early develu|»mont of villi in characleristio of the human chorion {Fig. 40 and Plate II, Fig. 6). At first the

TlQ

19.— H

man ovum o

mb'jutlU'L'lve

Fl'

('•

—Front view of

Keioherl), « 

U vie*.

FIK.4B.

la tKitb ncurui the

illi are limlled 1

rtfH

rll

lea vine th

villi, either covering the entire surface of the chorion or leaving the two opposite poles free, are of uniform size ; in the latter half of the first month, however, there begins to be a differentiation into a r^on containing smaller, and one hiiving larger and more nnmerous, pmjections. The difference between the two area* becoming more marked, the relatively smooth part of the membrane, possessed of rudimentary villi, is designated the chorion love, while the region with well-develoiHsd villous projections is distinguished as the chorion frondoBom (Plate III., Figs. 1 and 2 the latter a jqim-es of the uterus and close relation with the mucous membrane i the fetal part of the placenta.

The villi in their earlier condition are somewhat club-shaped elevations, which later become branched to form secondary villi. Each fully developed villus consists of a core of mesoblast, covered with ectodermic epithelium and containing blood-vessels (Fig. 54 and Plate IV., a). Their microscopic appearance is so characteristic that they afford a means of positively determining whether a mass discharged from the uterus is or is not a product of conception. The further alterations in the villi as well as in the trophoblast in general, including the syncytium, will be considered in Chapter VI.

A chorion is present, as a rule, in those animals whose embryos develop witliin the uterus ; this would include the entire class Mammalia, with the exception of the monotremes, wliose eggs undergo extra-uterine development, and the marsupials, whose embryos, though nourislied in the womb, never acquire villi on the serosa, nutriment being absorbed by simple contact of the latter with the uterine mucous membrane. the Mammalia are therefore divided into the Achoria, comprising the monotremes and marsupials, and the Ohoriata, including all other mammals.

+++++++++++++++++++++++++

CHAPTER VI.

THE DECIDU>E AND THE EMBEDDING OF THE OVUM. THE PLACENTA. THE UMBILICAL CORD.

THE DECIDU-C AND THE EMBEDDING OF THE OVUM.

The decidnn (deciduous ur cadncons membranes) are the hy[)ertrophie(l mucosa i»f the uterus so dcvelojwd as to form not only a lining for the uterine t-avity, liut also an envelope enclosing the ovum, and a specially all^reil part which serves as a bond of connection between the ovnin and the womb.

During tlic four or five days preceding lucnslruation. the socalled constructive stage of the mensiruiil eytle, the miienus membrane of the womb liecomes mnch thickened and unusually vascular, the purpose of these changes being evidently the preparation of the uterus for the reception of the ovum in the event of impregnation. If impregnation has not occurred, the thickcueil mucosa, the decidua menstmalis, is in great part easi off ns a purt i^f (he menstrual discharge; if, on the other I, conception has taken place, ^e raucous membrane undergoes I greater hyj^ertrophy. On scc, it is seen lo consist of a super1 compact stratum and a deeper b^'™"""*' ^nrr layer rcjKising directly upon the muscular wall of the {.Uterus, In the compact layer are the necks of the much enlarged uterine glands, while in the spongy layer are their greatly branched and often tortuous bodies (Fig. 51). The tortuosity and division of the deeper extremities of the glands produce the characteristic appearance of a section of the spongy stratum.

jl.— Crnan Beellon through iici.iia nicttibrane of Iho Hi the begliiiilng nf preg^ (aner Kiindral knd Bn

The alterations necessary to convert the menstrual decidua into the decidusB of pregnancy take place in part while the ovum is still in the Fallopian tube ; when it reaches the uterus it becomes attached to the mucous membrane of the latter, usually along the upper part of the posterior wall. A portion of the mucous membrane eventually comes to enclose the ovum as in a distinct envelope (Plate V., Fig. 1). The part of the uterine mucosa which thus surrounds the ovum is the decidua reflexa; the part still lining the cavity of the womb 13 the decidua vera ; the part that is in contact with the chorion frondosum is the decidua serotina. The decidua sor\>tina afterward becomes the maternal part of the placenta, iMtimatoly uniting with the chorion frondosum.

r«til recontlv it was believed that the ovum became imnUnUnl ti/K)n the surface of the mucosa, and that the latter irr\^\v up nr\>und and over it to form the decidua reflexa. VUi^ tluH^ry has Ik^ou completely set aside by the recent m\\v<turHtions of IVters of Vienna, whose results have been v\^«Knm>i by Webster of Chicago. Peters' observations \iv*v uu^\io u|H\n the gravid uterus of a suicide, the ovum ts^a^ v^^nln^UUnl in a triangular i>rominence on the upi)er »Hxx^<%»* ^\<:<\m of the i>osterior uterine wall. The ovum »Mxs«.xut^>J »u tlmv diameters respectively 1.6, 0.8, and 0.9 »wu». »w vN^^mKAUnl HS^^ iHMUg three or four days.

tX^ ■im* tt i t lft'if ^Iht OTum (Plate IV.), or its sinking into

Ik u,4x>s.^*, X v^uK^kly mHH>mplished by the c-osion of the

. , s ;4w ;;<: 'u>> t^ xv|^ iho hitter, presumably by the phagocytic ^. u .. i »iK M^»^4^UxU*t. Actual erosion is evident from

.», . <..;.>> ^4 »^>^ xnH^ivv epithelium at this place. The .»..^ KUH*^^at vM^x ivlation with the deeper layers of

\ , X ,v ♦ *K^ vN^K^^ v^' tW excavation are undermined,

.1, v.. .a '^ jmhK ww^hI by the mucosa, the area

i. . v.;vvl \iu^ v\vu|^\s) by an organized blood clot.

the tisane ftmgns (Plate IV.). The overhanging edges of the excavation constitute the beginning of the reflexa, which is obviously, therefore, not produced by the upgrowth of a circular fold of mucosa. The trophoblast strands or early villi extend toward and into the serotina, to which some of them become attached. It is thought by Webster that they may absorb fluid and imtriment, and that by phagocytic action they open up the blood-spaces of the serotina, thus bringing them into communication with the lacuna? of the trophoblast.

The blood-lacunse of the trophoblast form a system of intercommunicating spaces, the beginnings of the later intervillous spaces of the placenta ; they are filled with maternal blood from the serotina, and are lined with syncytium (Fig. 63), the latter being thin in places and resembling an endothelium. There is no extension, however, of the endothelium of the serotinal vessels, either upon the villi or into the spaces (Peters and Webster).

The ssmcytiiun is the more or less irregular layer of nucleated protoplasm which appears upon the surface of the ovum toward the end of the first week, lining the trophoblast lacunae, and later |>enetrating as irregular masses into the serotina, where it is found until the end of pregnancy. No traces of it are found on the veni after the sixth week. The origin of the syncytium has long been in dispute. Peters has shown that it results from the tninsformation of the superficial part of the trophoblast, probably from contact of the latter \\\i\\ maternal blood, which, he thinks, exercises a blending influence upon the trophoblast cells, so that as individual cells they disappear, the result being a non-cellular but nucleated protoplasm. Peters also believes that corpuscles of the maternal blood are appro])riated by the syncytium, and that the latter, covering villi and chorion as it does, has something to do with the interchange of nutriment and waste products between the maternal blood and the ovum. What remains of the early trophoblast after the formation of the svncvtium is the layer of Langhans.

The ciliated epithelium of the uterine mucous membrane disappears by the end of the first month of pregnancy (Minot) ; somewhat later, that of the uterine glands is also lost. By the end of the fifth month the fetus and its appendages have increased in size to such an extent that they completely fill the cavity of the womb, and the space between the vera and the reflexa is obliterated. After the second month the vera becomes progressively thinner and the reflexa undergoes degenerative changes to such a degree that by the end of pregnancy merely remnants of it are present.* By the sixth month the vera is intimately blended with the chorion.

THE PLACENTA.

The placenta, in certain groups of mammals, including man, is the organ of nutrition for the fetus during about the latter two-thirds of the period of gestation. In man, it is a discoid structure, attached by one surface to the wall of the womb, and connected on its opiH)site aspect with the fetus through the medium of the umbilical cord.

The human placenta represents the highest specialization of an apparatus for bringing the fetal blood into intimate relation with the blood of the mother. In eggs that develop otit^ide of the body of the mother, such as those of reptiles, bink, and the lowest mammals, the Monotremata, the ^nviiuj ombrvo necessarily acquires no connection with tl^* uterine mucosa, but draws upon its original dower of outriuu^nt, the deutoplasm, until its development is compU»t\xL wht^n it bn»aks through the stiell and seeks its own Kkh{ : in ilu>i** srr*>U[>s the chorion develops no villi. In Uu> Miar^ipwl^ a srn>up of mammals higher than the monouvMuvx^ \\w ovum* although developing in the uterus, forms IK* V U»^t> v^>nmvtJo« with it, but obtains its nourishment bv viiMjsv utiMiMttort tr\mi the uterine mucosa. On the other i,i.«xi, u u\ iKUtuuHls higher than monotremes and marsu>,;.^ \w iK'i-^oii is vlistinguished by the presence of villi

■ ii. vvHx.» »K *»uuM» pUwnta and the non-villous chorion

. iu t4,..^x4iS4*;v svi^tniit ^rn^lations exist; for example, in

■'^.^ ^va,<.v^ uKi '^i.Hm^ vnUvrs^ there is no proper placenta,

99

expiiUinii of the fetus. In the Cariiivora tbe pliiuenia has the form of a zone or ring — placenta zonaria — while In man anil wrtain iilliwl in.iinmals, as opes, rodents, and some others, it is iliseoid in shape — placenta diacoidea.

The tLoman placenta is formed in the third month of pregnancy; since it rrsiilts from the union of the chorioa frondosum with tlic decidua serotina, it consists of a fetal and a matemal part.

Our conceptions of the development of the placenta miiet be modiHc<l to accord with recent investigations. The e

lx?<]<liii;rof thcnviim in the uterine mucosa and the modifications iM-curni)<; in the <-liiirion, including the };ru\vtli of iti> villi and its ditli-ri'ntlution into the ehoriiiii fnmdusun) and thf cliorion leve, have Ik-cii ci)nsidered nlwve. It will be r<;(-:tlli.-d iliiit the uvuni e:its ii.s way, aH it were, into the iiiwir-n, tliu-i <-aii>inf: th<' sii|terfieiiil layers of the latter to di.s»|i|iear at ihe >\w of implantation. It is [tossihiy this jiroeos of i-rosion n|>oii the jKirt of the fend trophoIilaKt thai o|ieii~ u]) llie :di-<-:idy ililati'd caiiillarie!; of the s<.'n)tinu — the siniiseB — and idlows the nialeriiiil hliMHl aeeesK to the hlo.-1-hi.-iuia- of the li-oi.hol.lasl. wlure it thus Imthes the jiriniiiive villi. Since the entire trojilmhlaiit is vaeiiolated,

Pn. TA— fthcnwIlF rrv «j.. ■iidfithi'lliim :

lln- niati-mal hUfKl sit this time — the lirsl week — is hroiitrht iiilo relation with the whole surface of the <'horii>n. lii the lail-f l.ulf of llie first month the distiiuMiou li.twcen ehoiion li'.i.'l'»-iitij and ehurion leve Itejiins to lie matiifcst, the villi •/ lt» miKT irmdiiaUy retn^rading until, in the sixth week, vui- i,Ti j^HAilv degenerated, many of iheni W\n\t without

'■ * : ii; .J*' M^ •■Ji'irion frondosnm inei-ease in si/e. nnmlier,

-.* ■■M.,)»n,.f wntu- of thorn acquiring jitlaelnucnt to the

- ■ •«• .re ui- tt.truW.mteringthei*<'n)tina] hhioil sinnti^'s.

— -«..... .' u*^ -^.li vtntinues tliroughont im'unaney.

_ -^v.,.« -5wil«-«* in ibe oecond week, these heing

THE PLACENTA.

101

extensions of the allantoic blood-vessels. The syncytium of the chorionic laciinie, now the intervillous sp&cti.s, increases in quantity, and not only tines the spaces, but exists in the form of masses, some of which become attached to the semtina between the villi, while others penetrate into it, many l>eing found at the fourth month in the serotinal connective-tissue sjiaces. the decidua serotina (basal decidual in the first month is edematous ami hypercmic, presenting dilated capillaries and blood-spaces, many of which communicate with the intervillous spaces of the chorion. By the sixth weelt its surface epithelium is entirely lost, and the jiarts of its glanils contained in the compacta are to a great cxtutit tiblilerated. In

c leprvsenWdon of the dcvclopiui'iil <if Itii: placcnu. (altfir iTopholilut: •'<!i,. lyncyllum; £>i., cndulhelium ; Va^ eUl cBplllariea; d.>., ileclilual septum; F.b.. Qbrln;

the fourth month it is thinner, more irregular in thickness, contains less sinuses, and shows degeneration in the compacta, with many masses of syncytium. Toward the end of pregnancy the sinuses increase in sine, and the irregularity in thickness and the degeneration are more marked. The placenta at term is u discoid mass, in ttilu, but less flattened after its expulsion from the uterus. Its diameter is from 15 to 20 and its thickness from 3 to 4 centimeters. The uterine surface is convex and irregular, and is imperfectly divided into tnfts or cotyledons. The somewliut ooncave fetal surface, rather mottled, is covered by the loosely adherent amnion, and (iresents, usually near its center, the attachment of the umbilical cord. The maternal part of the placenta, the decidua serotma, is of varying thickness, in some places being absent. Its compact layer shows fibrinous degeneration, very few traces of glands, and no epithelium. The blood-spaces, lined with endothelium and representing greatly dilated capillaries, are in communication with the intervillous spaces of the fetal placenta (Plate IV., a). Scattered throughout the serotina are masses of syncytium. In the shed placenta there is very little of the serotina, since separation takes place through the compa^ta, the spongiosa and a part of the compactii remaining upon the uterine wall.

The fetal part comprises almost the entire thickness of the cast-off placenta. It is made up of villi* of all ages and sizes springing from the chorion (some attaclied distally to the serotina, others projecting free into the intervillous spaces), and of masses of syncytium attached to both villi and chorion (Plate IV., a). The intervillous spaces are a system of intercommunicating cavities through which the maternal blood circulates and which are in communication with the blood-spaces of the serotina. Elevations of the serotina between the villi constitute the septa placentae. The so-called marginal sinus at the periphery of the placenta is merely a system of intervillous spaces that intercommunicate more freelv because of the relative paucity and small size of the villi in this region.

The site of attachment of the placenta to the uterus is usually the upper part of the posterior wall. Under certain circumstances it may become attached lower down, even extending partly or wholly over the mouth of the womb, constituting then the condition known as placenta prsevia.

THE UMBILICAL CORD.

The blood-vessels through which the fetal blood finds its way from the fetus to the placenta and back again to the fetus, together with the atrophic vestiges of certain structures associated with the development of these vessels, constitute the structure known as the umbilical cord. In considering the growth of the human allantois it was pointed out that the

• For structure of villi, see pages 93, 94.

Diagrammatic reprcsenlatlon or ri hkltuf drat week; 2, ■ few dayi lalet; dcBned (WeL»lcrj : n. feUI meiDblut, i iolo Irophoblut (talks in I. nctual e reduced In i and.eonnltatlng ben th In l.enlirgad in

, a tew month owing Indication

Ui diTidiia: I. in latter

. when pli [>r beginning extension

3: b, trophciblaiit. being larei or l.angh«nii; c. tmphoblul lacuna ■pace ; d. lyncytlum, §etin in its

e. dfcidua: /. maturnal blmdnlnua: g, cndotbcUain lining matumal t. cplblMtlc covering of eord; f. amniotic eplbliut; J. nmbUieal

meanbloal

m. extension a/ decldua an unde

Sf obarlon at edge ur piocvnta ; n, la

latter structure, as it grows from the ventral wall of the guttract into the so-called allantoic or alxloniinal stalk, bci'omcs the seat of development of the two allantoic arteries and of an equal number of allantoic veins. With the metamorphosis of the chorion frondosum into the fetal placenta, the abdominal stalk becomes more slender and at the same time; much elongated, and the allantoic blo<Ml-v(*ss(>ls are henceforth the Tnnli11ic4il vasBels. The two umbilic^il veins fuse, so that, at birth and for somc^ time before, there is but one vein, though there are still two arteries. The nmbilical vein, entering the body of the fetus through the umbilicus, passes diro<'tly to the under surface of the liver, when^ it uuitt^s with the fetal portal vein and gives oif* a bnineh of eoiumunieation, the dnctos vanosiis, to the inferior vena <*ava, aft<*r wlii<*li it enters the liver through the transverse fissure. The umbilical arteries, whose intni-embryoni<' jxirtions are (^iIIcmI the hypogastric arteries, are the direct coutinuatious of tli<; su]MTior vesical arteries of adult anatomy. They l(*avc the binly of the fetus at the umbilicus.

The umbilical conl, while e<»nsisting essentially of the three blood-vessels nu^ntioned, continus also the remnant of the allantoic stalk and of the umbilical vesicle, these stnu^tures being surrounded an«l h(;l<l together by a <juantity of embryonic connective tissue, tli(» jelly of Wharton, whi<*li makes up the chief ])art of the mass of the eord ; upon the surface is a Iay<'r of epitlu'lium, <M)ntiiiuous, at tlu^ <Iistal end of the cord, with tli<; e])ithelium of tlu; amnion.

The umbiliejil eonl has an average l(»ngth of 5.") cni., or 22 inches, but varies Ix'tween the extremes of lo ('in. (<> in('hes) and IGO em. (G4 inches); its thickness is about 1.5 em. (J inch). The eord ])res(jnts tin* ap])earan(u^ of l)eing sj>irally twisted; it is prol)able, however, that the ap{>eanincc of torsion is conferred l)y tlu^ sj>inil or coiled arrangement of its arteries, due to their excessive growth, rather than by a twist of its entire mass. There mav be one or more true knots in the cord, produced by the slipping of the f(»tus through a loop.

The position of attachment of the (^ord to the placenta is usually near, but seldom exactly in, the center of the fetal surface of that organ ; rarely it may be found attached to its edge, and still more rarely to the fetal membranes themselves at some little distance from the edge of the placenta, with which, in the latter case, it is connected by its bloodvcwhjIh.

The great length of the human umbilical cord is thought to Im5 du(i to the relatively large quantity of amniotic fluid preH<*nt in the human subject.

Aflor birth, the portions of the hypogastric arteries extending from (h(? upper ])art of the lateral wall of the bladder to the unibilic'UH undergo atrophy, becoming impervious fibrous nirdn ; the intni-abdominal part of the umbilical vein likewine becomes atrophic and impervious, constituting the M)*<«itlled round ligament of the liver.

KliLATIONS OP THE PETAL MEMBRANES AT BIRTH.

When the amniotic fluid attains its maximum bulk — at about the end of the sixth month — it requires so much space thiit it. preHM(>H the amniotic membrane closely against the ehoHou, which IntttT, covered by the remnants of the reflexa, \n in turn f<»r<H'<l into intimate relation with the vera (IMute V.)' At t(*rm the vem and chorion have become pmctieully one menil)rane. The amnion, while adhering to the inner nurfurc of the chorion, is so loosely associated with the hitter that it may be peeled ott* from it. The membnines, which (M)nstitute a fluid-filled sac surrounding the fetuH, are rupture«l by the contractions of the uterus at some time during parturition. Through this rent the /;hild is forc'ed chn'ing birth, the placentii and the membranes remaining behind. After the expulsion of the child, the vera and the phieenta <h'tac'h themselves from the uterine wall, and, with the <^horiou and the amnion, constitute the afterbirth, whieii is expelled shortly after the expulsion of the child. The separation of the; after-birth takes place in the compact layer resjM'etively of the dwidua vera and of the utt»rine phieenta. The s|M)ngy layer and what remains of the com|)acta serve for the regeneration of the uterine mucosa.

fl

!l

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CHAPTER VII.

THE FURTHER DEVELOPMENT OF THE EXTERNAL

FORM OF THE BODY.

Having traced the growth of the germ to the time when the body of the embryo becomes definitely diiferentiated from the embryonic a})|)endages or fetal membranes, the development of the individual organs and tissues may be taken up. The discussion of this latter subject, especially of that part of it pertaining to the structures on the exterior of the body, involves a consideration of the external form of the embryo and fetus during the successive stages of growth.

In the preceding chapters it was pointed out that the cells of the segmented ovum arranged themselves in such a manner as to form a hollow vesicle, the blastodermic vesicle (Plate I.) ; that this vesicle, having at first a single-layered wall, came to consist of two layers of cells, the ectoderm and the entoderm ; and that, finally, a third, intervening layer, the mesoderm, made its appearance. It was shown, further, that the thickened portion of the vesicle wall, the embryonic area, became more and more differentiated from the remainder, and that, by certain processes of folding, this area was made to assume the definite form of the embryonic body, while from the other parts of the vesicle-walls the fetal membranes were produced (Plate II.). It may be well to remind the reader again that when the body of the embryo has become closed off from the fetal membranes, this body is an irregularly tubular structure whose walls are the somatopleure and whose enclosed space is the body-cavity, and that within it are two other tubes, a larger, the gut-tract, formed by the splanchnopleure, and a smaller ectodermic tube, the neural canal.

105

lOG TEXr-liOOK OF EMBRYOLOGY.

While, as a inattor of coiivcniciKjo, tho (]('scri]>tion of the individual organs is taken up after tracing the course of development to this stage, it should he borne in mind that the rudiments of some of them are already distinguishable before the germ-layers become infolded to form the IxKly-wall and the gut-tract. It will facilitate a comprehension of the gi'ueral principles concerne<l in the origin of the different |>Jirts of the body to R»fer to the tabulated statement of the derivativi»s of the three primary germ-layers as presenteil in Chapter III.

In ciMisidering the external form of the prcnluct of conception, one may adopt the classification of I lis, referred to in the first chapter. This author divides the jHTiod of development into thn»i» stages, of which the ///W, the stage of the onuiuor the blastodermic stage, comprises the first an<l second we^ks of inira-uterine gri»vth ; the Htroiufy the stage of the «BihiTO« extends frimi the seci>nd to the fifth wi»ek ; and the nhtnL or tte fetal stage, includes the time bi^twwn the fifth w<^k aud the end of gestation.

THE STAGE OF THE OVUM.

Durii^ :ht:* t'»rtnii:lit allotteil to this first staire of develop invut nxMT chie various changi»< l\v whirh the impn»gnateil

•\'im H'»tui:>f^ th-.' fi^rm of a hollow <phen\ de<ignat«»<l the

fnoi-vtiiiii- 'i* ■^i:i>ciHK'rTnir vehicle. Tlu* M-rir^of tnin^torma >Mi> wa^ u»'ii Fi"i*'rtb^il in Chapter II. In ihi> pla«v it will

'^ ^ttiKi:««iii I' r'vr c.» tht* external ch:inictrr< of the blasto i«gHK «*»a» t> A'CVCcxl in Fisr^. 4!>, •"»♦», whi.*h ri*pre<tMit

was estimated tu be alitml twelve days old. Its fonn van that of a spliore' snmewliut fijittf nod, its short and long diamot'.Ts inciisurins resiwcti vi'ly 3.3 mm. und 5.5 mm. T!ie flattened Bur&ces were t^llloolll, ^vhile the equatorial sons wns

!.' ;-. hloiiii-iiLii Liiiij( nil ilif iiiiii-r iMiliir iwrlloii of the ftiMriimlB

l'.E. uledliG epiLhulluiii^ Oii., duUdiut reQeia: TV, Inipbobliul; (b, nuKriut oaplllary: r)r, gland of uterine muMwa: /U.Jl.UciiaicIn (he traphoblail, conUlnIng malemil blood: K.A, nits' of embryil; Om/i. itvcldus rtuinparta : if, fbUI mEdohlut : L'.Z. liiti!rgliiu<liil*r llMue ur mticcwi. In which enrly dccidunl n'lln are

beset with villi istic of the hill

Tile I'arlv apiieiir.i

i is trharacter

THE STAGE OF THE EMBRYO.

It 13 during the early jKirt of the second stage, at about the fourteenth day, that the somatopleuric layer of the lilastodermic vesicle becomes folded in to prodtiee the walls of the

erabrvouic body. Fig, 67 allows a human etnbrvo of about the tifteenth day, wbose form ia as yet imnerfectiy differentiate, the ventral wall of tlio bixly Iwing incomplete, tiince the giit-trart Is tttitl in wmimimication with the iimbiliCftl viwioie throughout almost the entire lungth of the embryo. The Iwck and sides of the embn-o arp enveloped by the unnlon, and the dorsal outline is concave. The caudal pole

fM, M~HuHiiiii*itibrri>ifrKlii>ul (liu thltlvenlh asy (Rti) Tbemudiit lll>WUll>r|ri>liouiiii*Dtvil trlUi Ihv tilutcidiinnlc! vmicle bj miianBof tlii! alid ku wIIuhMi' lUUi Ui» ainnlan nl randy romplrtcly endtoiri the vtghryn.a tvTtiH iltHlllnHiiiiii^iiminiinlrBli-i throuKhoUl th«it«lvt partofthi: mllnl i

U mvn t« \\f winni't'twl by means of the ullantoip sliilk with ninu, which Intter Btnictiire, however, is not

th.i

iilllv.

i'fl>iv.(>nliil ill lliK rijrnre. The concavity of the dorsal ontliiu> in |Ht>nliiii- (.1 the hiiman embryo of this stage. The (luvi<lopini<n( III' thiT Iicud i'h eUmcly iiHB0ciat*'d wilb the dilatalitui uf \\w .'cphaiic end of the neiinil hibe and the subse(picnt divi-i.iH of thin dilated extremity into the thrt^ primary bmln-viwioh'M, tho forc-bmin, the mid-bruin, and the hindbmin. The oral pit, the Brut indication of ibe future mouth, ia )>rvMii]t in the curly part of this stage ; it is a depression

THE STAGE OF THE EMBRYO. Ill

the rudiments respectively of the crystalline lens and of the membranous internal ear; at this time also the visceral arches and clefts first become distinguishable. On the twenty-first day, the rudiments of the limbs appear as little bud-like processes springing from the trunk. The conspicuous projection on the ventral surface between the now almost completed yolk-sac and the cephalic end of the body is produced by the primitive heart (Fig. 59, 10, 11, and 12).

Until the twenty-first day the ombrvonic body is erect. Between the twenty-first and twenty- third days a marked alteration in the appearance of the germ is brought about by a pronounced bending of the long axis of the embryonic body (Fig. 59). The degree of curvature is such that the caudal and cephalic extremities overlap. The flexion reaches its maximum degree by the twenty-third day. The curved dorsal outline is referable to four well-marked flexions, the position of the most anterior, or cephalic flexure, corresponding to that of the future sella turcica and being indicated by the projection of the mid-brain vesicle (Fig. (>2, III.) ; at this point the anterior part of the head is bent almost sufficiently to form a right angle with tho posterior half. A second or cervical flexure is found in the future neck-region, while further caudad are seen the less pronounced dorsal and coccygeal curves.

The fourth week marks the period of the most active growth of the embryo. Afler the twenty-third day, the body as a whole uncoils somewhat, although in the latter half of the fourth week the individual flexures noted above become more conspicuous.

The Visceral Arches and Clefts. — The visceral arches, with the intervening visceral clefts, constitute a conspicuous feature of the extemal appearance of the embryo during this stage. These arches are a series of fivx» approximately parallel ridges appearing upon each side of the future neck-region and extending obliquely downward and forward toward the ventral surface of the embryo (Figs. f)0 and 62). Tho four furrows lying between the five visceral arches are the visceral clefts. A coronal section of the neck

UTa)u||Ulur ui prltntllvu l>sai

111* tu tkutUt: urtlii's (U^i ' "^- ">'■< uaxlltorr and mandlbuUr 'ml anh : o l-a IV, Bnl to Iburth wirlle anhiM : fv. cc. priuil* liial vain*; dC. fliicl "f Cuvlur; al. r. ■trioai and ventricle of iii'llin" •■a: ni. da. vtntral «nd donal anrtK: mi, oi, optic end

THE STAGE OF THE EMBRYO.

113

region (Fig. 61) — a section in a plane parallel with the ventral surface — shows that the furrows seen on the Dctodemiie surface correspond in position to a like number of deei>er grooves on the inner or entodermic surface. The inner furrows are out^ptK^ketiiigs of the entoderm lining the pliaryn^real region cif tiie furc-gut ; they are referred to as the pharyngireal poucheB or throat-pockets to distinguish them from the outer clefts. At the iiottoni of the clefts tiie ectoderm is in contact with the entoilerra, the meaoderra being absent; these two layers constitute the closliig membrane. The visceral arches or ridges consist of tliickened masses of niesodermic tissue covered outwardly and inwardly respectively

FlQ.

phsrynp'Bl end of gut-lrsot from behind (froi

eniljryo ot a.iinm.: B, of 4.ffi mm. (about 2510

ceni ftirrawa; I', bIdus privcerTlHlls, compriiing Ilifrd and tuurth oi

, *. !l, (, rlMoral Bwhos. each witb lis »jB™™i-ureh vesasl

B. lubtrc

7, orinpo of lirj-Qi ; *, pulmonnry evagioatlou.

by the ectoderm anil the entoderm. Each arch contains an artery, the visceral-arcli vesBel. these five pairs of visceralarch vessels arise by :i common stem, the tnmcns arteriosus, from the primitive Iieurt.'

The morphological significance of the visceral arcbes and clefts may be a]ipreeiated by a comparison of the conditions obtaining in lower types. While in birds and mammals the

' Fur U11 account of llii: luetaiiiorphusis of the viac!«rnl-Hrc]i vessels inlo the udiilL arlcriee uf llie ihniit mid neck llie render in referred Lu Chapter

number of the lAetia 13 four, in reptiles, amphibiaiis, and bony fishes, Hve clefts appear, and In some fi.shes (selachians) the number is six. In alt aqnatiu verteiirates, the thio epithelial closing membranes nipture, thus establishing communications between t!ie alimentary tract ami the exterior, tlirough which ojienings water passes in and out. The margins of the cleibi — except the first or hyoraandibular cleft — become the scat of a rich supplv i>f capillary blood-vessels, the blood of which obtains oxygen fmm the water and yields to the latter its carbon dioxid; while the visceral arches, excluding the first and second, become known in these classes as brancbial arches from their producing bony arches which support the branchiEe or gilis. With the exceptions noted, the viseenil arches and elerts with their capillary plexusea therefore functionate in these classes as a respiratory ap»,J pa rat us.

When, in the course of evolution, certain of the vert«-i brates assume an aerial existenw, in consequence of whicbl they acquire a breathing mechanism adapted to such a model of life, the respiratory function of the clet^s or branchis^a ceases, and they either disapjiear entirely or constitute merely.! rudimentary structures of the adult. The so-called clefts in f man aie never actual openings, the closing membrane always- 1 being present (His, KoUiker, Piersol, Born), To express the i morphology of the visceral clefts* briefly, they are permanent J structures in flshes and in tailed Amphibia; they are present ' during the larval stage of other Amphibia, while in bird» \ and mammals they are found only in embryonic life.

The growth of the visceral arches and clefts bears an intimate relation to the difTerentiatiou of the head- and the neckr^ions of theembryo. They first make their appearance at about the twenty-third ilay and attain their greatest development by the end of the fourth we^k. Both the arches and the clefts appear earliest and are best developed at the cephalic end of the scries, the fifth arch being exceedingly illdefinecl. During the fifth week the obliteration of the arches and clefts as such begins, since certain of them become metamorphosed into permanent structure;^ wliile the 1 undergo regression.

The Metamorphosis of the Visceral Arches and Olefts. — The first visceral arch becomes (livide<l into an upper i)art, the maxillary arch, and a lower ])ortion, the mandibular or jaw-arch (Fig. 62). The maxillary arches or processes of the two sides unite^ at their anterior ends, with the intervening nasofrontal process (Fig. 67, and in tliis way is formed the upper l)oundary of the mouth-cavity ; the mandibular processes become joined with each other anteriorly and constitute the inferior boundary of this cavity. The maxillary processes become the superior maxillie, while the mandibular [)rocesses l)ecome the lower jaws. The mesodermic core of the mass of tissue constituting the mandibular arch divides into three sections, of which the two situated at the proximal end of the arch are quite small and give rise respectively to the incus and the greater part of the malleus ; the large distal segment is a slender cartilaginous rod, Meckel's cartilage, whose proximal extremity becomes the processus gracilis of the malleus (see Chapter XVIII.).

The second visceral, or anterior hyoid arch becomes obliterated as such, although a bar of cartilage which it contains — Beichert's cartilage — gives rise by its proximal extremity to the stapes,^ while the remaining portion becomes metamorphosed into the styloid process, the stylohyoid ligament, and the lesser cornu of the hyoid bone.

The third or posterior hyoid arch, which corresponds with the first branchial arch of fishes, likewise loses its identity as a surface marking, while the bar of cartilage it contains becomes the body and greater cornu of the hyoid bone.

The fourth and fifth arches coalesce with the adjacent tissues, producing no special structures.

The first outer cleft, known as the hyomandibular cleft, suffers obliteration except at its dorsal extremity, where the tissues forming its margins produce the external ear. The remaining three outer clefts disappear in the following manner : the fourth outer cleft becomes covered and hidden by the fourth arch, and the third and second clefts are successively

• Reichert, (iegenbaur, Ilertwig ; or to the ring of the stapes according

to Salensky, (jradenigo, and Rabl.

tiuried by the growth of the third and second arches. The sinking-in of the lower arches and clefts (Fig. 61) results in

fc. »•.»». 1-^:1. J", limb*;. iJ.."11«nlolc »li.lk:rA. vil

« on the lateral surface of the ^ft^^i^^B*^«F^ 6*2, »p), ^v)lieh snUeqiienlly

is made to disappear by the coalescence of its edges. Occasionally this sinus, instead of becoming completely obliterate<l, persists, and the thin layer of tissue forming its bottom ruptures — possibly spontaneously or perhaps more probably as the result of exploratory probing — constituting the anomaly known as cervical fistula. Such a fistula establishes an opening into the esophagus.

The first inner cleft or first pharyngeal pouch becomes metamorphosed into the middle ear and the Eustachian tube, the closing membrane, which separates it from the outer cleft, forming the membrana tympani. The second pharyngeal pouches produce no special structures, but the adjacent tissues give rise to the epithelial parts of the middle lobe of the thyroid body and to the posterior third of the tongue, in the manner more fully indicated on pp. 143 and 226. The third inner cleft produces the thymus body, while from the fourth results the lateral lobes of the thyroid bod v.

The configuration of the face, depending as it does so largely upon the development of the boundaries of the nose and of the mouth, is closely associated with the growth of the first pair of visceral arches. The earliest indication of the mouth, the oral pit, appears at about the twelfth day as a shallow depression on the ventral surface of the embryonic body l)etween the fore-brain vesicle and the prominence caused by the primitive heart (Fig. 59, 3 to 5). This depression is deepened by the growth of the tissues surrounding it, as also by the flexure of the head, which occurs at the twentv-first dav. In the third week, therefore, the oral pit is a five-sided fossa, being bounded above by the nasofrontal process, which has grown down from the elevation of the fore-brain, laterally by the maxillary processes, and below by the mandibular arches (Fig. 67, ^1). The pharsmgeal membrane, which consists of opposed ectoderm and entoderm and which separates the primitive oral cavity from the gut-tract (Fig. 66, rA), ruptures at the time of the appearance of the third branchial arch.

By the end of the third week, the communication between the yolk-sac and the gut-tract has become reduced to the relatively small vitelline duct. At the twenty-fifth day the

enihrvo j»n'sc»iits a well-(lov(*l()i>o(l tail. By tli<» termination of the fourth week the volk-sac has attained its maxiinum size, and the presence nf the s<nnite.s is indieateil hy transverse ])ai*:inel lines on the dorsal snrfaee of tlu^ IxkIv.

THE STAGE OF THE FETUS.

This sta^e odinprises tin* time between the beginning of the second inniith and the end of jircirnancy.

Dnrin^ the second month tiic rate of irrowth is far less nipid than in the jm'ccdinj^ stap*. The marked enrvatnre ol' the h»n^ axis (»f tlu* ImmIv jVradnMlly dimi>hes, the embryo assnming a more (;re<a [)osture. Owin^r t-> the partial disaj)|M*aninc(^ of the cervical tU'xure, the iiead l)econie< raised.

I>nrin«^ \\w. fifth week tiie vitelline duct is >een to bo lonjx iind slendei , the umbilical cord lias become longer and nion* snind and mav contain u coil of intestine ; the abdomen is very |)i*ominent, and in the neck-region is a characteristic dorsjii concavity. At tiiis time al>o the nasal pits Ikhmmuc conspicuous as depressions situated on either side of llu* nasofrontal process (Fig. (57j. Th<* nasofrontal jnvx'css ini':in\vldle undergtN'sdiiferentiation int«> the globular processes, which constitute the inner boundaries of the na<al ]»its, and the lateral frontal processes, which limit these tlepre^^ions exter

Vi« W llttmM»m\itytMiftl»ulrix weeks. cnlnrL'id ihr.-.- lii;., . \u.

\»\\\ a\u\ M'\»rAW\W'm fn>m the dcpn'ssicMi- tni- ih. . y..^. Aw twM\ \wu ftw hl\\\ iu coramuuiiration WUw \\\\\\ the

primitive oral cavity. The lacrimal groove is well-markeil at this stage, and tho external auditory meatus is indicated. The mandibles become united mesially at about the thirtyfourth day. The third and fourth gill-clefts have by this time disappeared in the cervical sinus. The paddle-like limbbuds have lengthened and present, at first, a division into two segmcHits, of which the distal is destined to become the hand or foot, while the proximal jwrtion undergoes segmentation a little later into the arm and forearm or thigh and leg ; by the thirty-second day, the hand, now showing differentiation into a thicker proximal and a thinner terminal part, exhibits the first traces of digitiition, in the form of parallel longitudinal markings which soon become grooves and, later, clefts. The develoj)ment of the upper extremities precedes that of the lower by twelve or fourteen days.

During the sixth week the head assumes more nearly its normal [)()sition, and for this reason the apparent length of the fetus is considerably increased, the dorsiil concavity in the neck-region being ahnost obliterated ; the rudiments of the eyelids and of the concha become recognizable, and the various parts of the face assume more definite shape. By the fortieth day the oral cavity has become separated from the nasal pits by the union of the nasofrontal process with the maxillary process(\s, and the external boundaries of the nostrils have become marked out by the meeting of each lat(^ral frontal j)rocess with the corresponding maxillary ])rocess. As a result of these changes, the nose, although still very broad, begins to assume characteristic form. During this week also the fingers are seen as separate outgrowths, while in the seventh week the rudiments of their nails become evident.

Toward the end of the second month — about the fiftieth to the fifty-third day — the toes are just beginning to separate, the protrusion of the intestine at the umbilicus is at its maximum, the palpebral conjunctiva separates from tiie cornea, and the rudimentary tail begins to disiippear.

The eighth week witnesses the total disaj)pea ranee of the free tail, the formation of the septum that divides the cloaca info the rectum and the f^nito-urinarv passage, and the presence of the project iiig genital tubercle with the accompanying genital folds and genital ridges. The external genitals as yet show, no distinction of sex. Fnini the end of the second month to the time of birtli, fetal growth is, in great measure, merely the further develojmient of organs already mapped out; it is held by many authoritifs, therefore, that if mal format ion.s are ever due to maternal impreasions, such impre.-isions could be oiierative only hi the event

of having been i-eceivcil prior to the eighth week of gestation.

Dnring the tJilrd month, the face, although definitely formed, still presents thick lips, a pointed chin, and a rather bniad and triangular nnse. At this time the Wtaha are wellformed and assume a eharaeteristie attitude, and the fiugiTs and toes are provided with imperfect nails. The external genitals, which, until the close of the second month, preserved the indifferent type, now begin to show sexual distinction.

In the fourtli month, a growth of fine hair, the lanngo, appear> npiwi llie sculp anfl some other parts of the body;

the anus ojiens ; the intestine rece^Iea within the abdomen; and the external generative organs present well-marked sexual characteristics.

The fifth month marks the inauguration of active fetal movements and the appearance of a more plentiful growth of colorless hair.

In the sixth month the fetal bo(]y becomes coaled with the Temix caseoaa, a modified sebaceous secretion whose func

tion is the protection of the epidermis fnmi maceration in the amniotic fluid. The eyebrows and eyelashes also appear about this time.

The aeventh month witnesses the appearance of the lanugo, orembryonal down, upon practically the entire surface of the body ; the testes of the male fetus are in the inguinal canal or at the internal abdominal ring; and the nails break through their epiilermal covering. Children born at the end of tlic seventli month niav survive, but usuallv thev do not.

In th(» eighth month the lanugo begins to disappear.

In the ninth month the testicles are found in the scrotum, while, in the ease of Xhv. femah*, the labia majora are in contact with each other. The contents of the intestinal canal, the meconium, consisting of intestinal and hepatic secretions mingled with epidermal cells and hairs swallowed bv the fditus, is now of a dark greenish color. The umbilicus is almost exactlv in the middle of the bo<lv.

The weight of the fetus at full term is fnmi 3 to 3.5 kilograms (from to 7 ]K)unds), the average* weight of the male child being about ten ounces greater than that of the female. While variati(ms from these figures are not uncommon, statements of excessive weight are to be received with reservation, since it has been found, ujxm careful observation by comjH?tent authorities, that the weight of a new-born infant rarely excenls ten poimds. The weight of the chihl, besides dej>ending u|M)n the ])hysical condition of both parents, is influenced bv the age of the mother, young wcmien having the smallest, and women between the ages of thirty and thirtyfive having the heaviest children ; by the nimiber of ])revious pn-gnancies, the weight being gn»aler with each succeeding iirt^nw»ev. pnividt^l the successive children are of the same Ti^.x :.r:d ar\* not lK>rn at t(K» short intervals ; ami also by the veir:.: Ga>>uiT' and height (FrankmhaiiMn) of the mother, Mv •^.*-' >:n:r a dinrt (juc. Min(>t bcH<*V('s that these vui'«i> T.f •iixx^ o|K*rate chiefly by prol(Hi<:iiiL^ or abbreviat-.-• T?r n:'"'» ti .:' 5:*Maiion, and that therefore the variati«»ns T T^^-u iL i^^V. Jirv* rt»fenible to tw«. prineipal causes — /,^..^.., ..^ ,1 '\^* s-^ ^\ birth, and variatit»n> in the rate of

Mggfc M lite li^ns Ht the time of birth ir* about oO

1^ jf ;u •jnbryo or fetus may l»e esti liftir**-*:-^ 'pcvuliar to each >tai:e a- above

.iici. * Hi;. 'iK- nde tbnnulatetl by JIaa-e. Me iiid of the liflh niMiitli, tin

square of the age in months equals the length in centimeters, while after the fifth montli, the length expressed in centimeters equals the age in months multiplied by five. Thus a fetus of four months would have a length of 16 centimeters; while one of six months would be 30 centimeters long. Hence, the age in months is the square root of the number expressing the length in centimeters; or, if the length exceeds 30 centimeters, the age in months is one-fifth of the length expressed in centimeters.

Reference has been made in Chapter I., page 40, to the relation between conception and menstruation, and to the manner of estimating the age of the product of gestation, based upon this relation.

+++++++++++++++++++++++++

CHAPTER VIII.

THE DEVELOPMENT OF THE CONNECTIVE TISSUES OF THE BODY AND OF THE LYMPHATIC SYSTEM.

THE CONNECTIVE TISSUES.

I'llK varioUHly iiKKlified forms of connective tissue distribiiti'd tlironj^hoiit the body, including such diversified tissues iiH thi^ l>lood and the lymph, areolar tissue, fibrous and elastic tiHHUr, adenoid tissue, tendon, cartilage, bone, and dentine, an \v«'ll an i\n* ('onnective-tissue stroma of various organs, all n^Mult from alt<»rations affecting the middle germ-layer or meioderm. As pointed out elsewhere (Chapter III.), the inner and the outer germ-layers are concerned in producing the epithelial Htruetures of the body (with the exception of the epithelium of the greater part of the genital apparatus and t»f the kidney aixl ureter), the ectoderm giving rise not only to the epithelium of the surface of the body, but also, by prnerHHeK of infolding, to such important structures as the eentrnl nervous system and the internal ear, while the entoderm dillerentiates into the epithelial parts of the respiratory and digestive systems with their associate<l glandular organs.

The proliferation of the cells of the mesoderm goes hand in hand with thedifVerentiations of the inner and outer germlayers, so that even at an early stage of development the middle germ-layer, besides having given rise to the mesothelium of the body-c^avity and to the primitive segments, C(mHtitutes a loose aggregation of cells that fill the spaces between the g<'rm-layers and spread about the deveh)ping embryonic organs. This primitive relation of the mesoderm i<^ tissue foreshadows its future office as the supporting fnunework not only of the bmly, but of the functionally active epithelial elements of the glands. Thus, the indifferent mesodermic tissue that comes to surround the notochord and the neural canal specializes into the spinal column and the brain-case ; while the parts of this tissue into which protrude the epithelial e vagi nations of the primitive alimentary canal — as, for example, the cvaginations which are the beginnings of its glandular organs, the liver and the pancreas — become intimately associated with these epithelial sacs and tubes to constitute the connective-tissue stroma and the vascular apparatus of the completed glands. All organs of the body, therefore, that have a connective-tissue constituent obtain it from the mesoderm. Owing to the varying degree of differentiation of the mesodermi(; elements in different localities there are formed tissues of widely different character. The most important factor in the production of these modifications is ihC' alteraiion of the intercellular svhstanccy as to whether it remains soft and homogeneous, whether it acquires a fibrillar or an elastic structure, or whether it becomes dense and hard, as in the case of cartilage and bone. The celh undergo comparatively little change, although, according to the kind of tissue produced, they come to be known respectively as connective-tissue cells, tendon-cells, cartilage-cells, or bone-cells.

The slightest degree of specialization results in the production of mucous tissue. In this case a reticulum is formed by the slender processes which the cells acquire, the spaces of the meshwork beinj' filled with the semifluid or semigelatinous intercellular substance.

A further alteration in the intercellular substance, whereby it acquires greater density and becomes permeated by bundles of fibers, some of which are highly elastic, results in the formation of areolar tissue. Preponderan(;e of the non-elastic fibrous element produces white fibrous tissue, while elastic tissue, such as predominates in the ligamentum nuchje, is formed if the elastic fibers are in excess. Further increase in the densitv of the intercellular material, with its accompanying conversion into bundles of non-elastic fibers having a characteristic regularity of arrangement, produces the struct

iin; of tendon. Wlion the intercellular substance gives rise to a scant amount of fihrous material and the cells become (li?*ten(le<l with oily or fatty matter, adipose tissue results.

A still greater degree of density of the intercellular subflt^incc givcffl the matrix of cartilage, the cells being enclosed in H])aceH, the lacunse, as the cartilage-cells. Partial differentiation int^» either fibrous or elastic bundles confers the r'hanicrter of eith<*r fibrous or elastic cartilage wy^n the pHKlnct.

Great cond<;nsati(m of the intercellular substance and its pennf.*ation with salts of lime, the cells being fixed in small «pare*^, results in the prcKlucticm of osseous tissue (see Chapter XVIII.).

Blood and lymph may be hK)ked upon as forms of connective Xx^uii in w^hieh the interc<;llular substanc^e is fluid, con^titnting the plasma, the cellular elements thus remaining free c<-II«, the blood- or the lymph-corpuscles. The development f>i br»th lymph and bl(KKl from the mesodermic elements .J4*rv*^ to l>ear out the comparisf>n.

The cadothelimn of the body is related with the connective -••^les jzenetirally as well as anatomically. Reference has ven iiaiie el?ewhere to the changes which occur in the in*^Hi»»rji:»' rt-ll^ that bound the body-cavity — the fissure

vr««>v>i Tie two layers into which the parietal plate of the iiK>!Miik«ni s?i:t> — to constitute the mesothelium of the body atvTT*-^ r*iese change? consist in the flattening of the cells

091 :*v«r ts»!*in.'.oc>^n of the characters of endothelium. ^•WtTr;;ir-\, T-u»n 'cr.^T smaller clefts are formed in the meso vi^iTTTX ->»«%.. ••\"^* which may l)e the beginnings of small -r«iMr.>fwv%. r -r bUi»xl-vessels, or of bursal or articular

•• — ■*. -n xw^wr-ttc ^tfiL< of these cavities also assume the

^, .,^^. , 4ftMl^HMrt of the serous membranes and of ^^. ..r^^ ij^jBifcr Innalt and thecal sacs may be

^ n:., loc^ ">^tt Slid about the origin of the

.^„,„^. -iv N^itfti^'^tvv^isi^ie stroma of the mem _ ^- H *nA»iiu'lmui rests, is simply a con ^^ . ^ .sv«v^«*»«^ :Min?Uifc vtf connective tissue.

THE DEVELOPMENT OF THE LYMPHATIC SYSTEM.

The solid elements of the lymphatic system — the " Ijrmphglands," the lympli-follicles, and the diffuse adenoid tissue — as well as the thsrmus body and the spleen, result from the specialization of mesodermic cells, while the lymph-vessels and the various Ijrmph-spaces of the economy — that is, the s(»rous sacs, joint-cavities, bursid and thecal cavities, subarachnoid and sulKlural spaces of the brain and spinal cord — are develo()ed by vacuolation or hollowing out of the mesoderm.

Definite knowledge is wanting as to many of the details of the genesis of the lymphatic system. The various lymphspaces precede the vessels and the adenoid tissue in development.

The Isrmpli-spaces result from clefts in the mesoderm, the earliest formed and most c<mspicuous space of this sort being the body-cavity or coelom. This large fissure develops, even before the differentiation of the body of the embryo, bv the coalescence of numerous small cavities that appear within the middle germ-layer. The body-cavity acquires more definite boundaries by the alteration of the mesodermic cells that border it into flattened endothelioid cells, the mesothelium of the body-ca,vity. AVhen, in the progress of development, the diaphragm and the pericardium are formed, the body-cavity is divided into the peritoneal cavity, the pleural sacs, and the pericardium. At a still later period, a diverticulum of the peritoneum protrudes, in the male fetus, through the inguinal canal into the scrotum to constitute the tunica vaginalis testis. The stomata of serous membranes are merely so many apertures of communication between the serous cavities, which are enormous lymph-spaces, and the lymphatic clefts contained within the stroma of the serous membrane, the clefts themselves being the beginnings of lymph- vessels.

The large Isnnph-sacs surrounding the brain and spinal cord, the subarachnoid and subdural spaces, as well as the spaces within the capsule of Tenon and the sheath of the

o|)ti«t n<'r\w, jiihI tIm; p«*rilyiii])liati(' spaces of tlio internal ear similarly ilevdop a-^ vaeiiolati<>ns <»f the nio.s<KU*rniio ti^>ue. '1'Im> .siiiie is true of the joint-cavities, bursal sacs, Hheaths of tendons, and the small lymph-clefts ioiinil in the areiilar tissue and throughout nio>t orir»nis.

The lymphatic vessels fir>t lornie«l,a<v«)r<lin<!: toO. Schultze,

are the siilM'titaMeoii> v<*>r»(*N, whi<*h are present in a iinnian

ehihryo of 2 to .'» <'in.,' aii<l at a -onu'what later perio«] the

Jei'prr vrsH'ls apprar. From the stuthes <if Sal)in" upon

piir <'ml»rvos it appear^, liowever, that the lar<rer ves>els

preenh* th<' smalh't*. Tiii^i ol»erver found that at the jnne tion t»f th<' sulielavian an<l jugular vc-ins of eaeh si<h» a sae

<»r lymph'hoart made ilsap|M*aranee, the oritiec In'ino: guarded

l»v a valve, and from th<'s<' >aes or hearts hninches arose

wich passi'd t«»\vard th*- skin, from wich hranehes a <reneral

hulieiitaneous net\\t»rk of ve-M-lr. aroH*. From eaeh lymj)h Hie a vessi'l m'ow.s tailward, the vessel tui the left side

rt'Mehin^ \\\v a«>rta and divi<lin^ there to form two thoraeie

diU'l^, wliieli afterwani unite into a sinirh* duet. F^recpiently

ihU fi'lal condition oi* two thoracic duets is indicated in the

liinnMii adult hv a douMc condititin of the du<*t tor a greater

or h'HM extent, the diK't sometimes dividinir and reunitinir

two or three tiine>; sometime^ it i*^ «]oul)]e at its termination

ill the Mi1iclav!:in vein.

Tin* two llniraei<* «luctr* hefore fu-ioii dilate at their <-audal

ixtivmiti^, ill the re^d«»n i»f the ki^imy, to form respeetively

twj /.ri/,/f»r»/f/ c/iy/, and a litth- fariher on miite with the

iw'i ywWlor lyinph-hearts or sa<'s, wich iiave meanwhile

.1. m\u\^A ill th- jimctitin of the s.-iati<* veins with tl ardiual

^el1^.. TIm^*' IsiHcr sMcs MiliMMpicutly lo-c all coniieeiion

,.,^1. d,. v.in^ rrniii whidi tlK-y .irrew. ()uton>wih. frmn

h... .Wr v«'^-'U ^'i'the hmphatic system -erve f^r its

...M.-..M. nn., the vi-reni aiitl the skin. As the primary

' ..,.. ..„ '..■.iMri- iliT Kntwii-krluntf-niMliichti- .1.- .M.u^l.vn

. ..... \ ...,.,.1 ■ «HMlM-«»ri>:in«»f the Ly'»p't» ^>"»' '"'■•' ^'»

..... ;,.... P^H.^1.' "f thf Kynn)h-lu*art< aiul Tlh.n. ir iMin in

lymph-sacs increase in length, but fail to correspondingly increase in calibre, they gradually become merged into the vessels.

The lymphoid or adenoid tissue is produced at a later date than the vessels. Observations upon the human lymph nodes seem to have been confined to the inguinal and lumbar nodes. The first indication of an inguinal node is seen in a 3 cm. embryo, in the shape of little aggregations of lymphoid cells that have migrated from the lymphatic cords or networks into a space hollowed out of the mesoderm. This nodule of lymphoid cells is isolated from the surrounding mesodermic elements by a fissure or space except at one point, the future hilom of the node, where strands of embryonal connective tissue connect it with the parent mesoderm. The retdcolum of the node appears later, as does also the capsule, the latter of which results from the condensation of the surrounding mesoderm.

The development of the spleen is considered with that of the alimentary system because of its relation to the evolution of the peritoneum, while the account of the development of the thymus will be found in the chapter on the respiratory system.

9

+++++++++++++++++++++++++

CHAPTER IX.

THE DEVELOPMENT OF THE FACE AND OF THE

MOUTH-CAVITY.

The evolution of the £ace depends so largely upon the growth of the parts concerned especially in the production of the mouth and nose that any account of its development must deal for the most part with the development of those structures. In tracing the earliest stages of facial growtli, it will be well to consider the face as a whole before proceeding to a detailed description of its several parts. If we seek the principles underlying the conformation of the face, we shall find that its apertures and chief cavities are merely so many provisions for bringing the central nervous system and the aliraentarv tract into relation with the outside world. It will be seen, for example, that certain small depressions apiH'ur U|H>n the surface ; that one of these, which is destined to l¥H^>me the mouth and the respiratory part of the nasal dviticss a-ssumes relationship with the alimentary tract and with Its offshi^ot, the respiratory system ; thatother depres^MtSs. whioh s«l>scquently develop into the olfactory parts >^< th^ iwfcal ohanilH^rs, come into relation with (»utgrowths •^•m tis^ hmiiu the olfactory bulbs ; and that still another '*l^nl%^•^*tt\;j^acttwltum In^comes the lens-vesicle, which likewise mvi>^ *»ttlk Att vHii^r\>wth from the brain to become a jmrt of ^ ^*-»f»4KMnt ^*ttp!*>H>npin* the eye.

^"N, ti-^ 'avp itt tht* ditferentiation of the face is the fort«i|tH>.r >4 iiKi <ml flilt» the earliest indication of the future •t%Hu^. ^V' vHiJ i^iUit^ ;jip(H'ars on the twelfth day, and con* * >iH«ut ,^*vH of wtiHlerra and entoderm, the meso*^«4k|^.iU<ai« U icL ^tuatcd on the ventral surface of

DEVELOPMENT OF FACE AND OF MOUTH-CAVITY. 131

the head-end of the embryo, which already presents the enlargement of the cerebral vesicles. The oral plate becomes relatively depressed by the upgrowth of the surrounding tissues, the fossa thus produced constituting the oial pit or Btomodsnin (Fig. 57). The oral plate is now the pbatyngeal » (Fig. 66). Reference to the sajpttal section will

Pro, W.— Medl«n setllon tlirough the henil of an emhryo rabbtt 6 mm. long (arttr MlhslkovlcB): rA. laerobrene bulvtiin stomndsuni and 10 ru^t. pharyngeal membrane (Rachenhauli; lip, place rrom which the hypophysla Is developed; A, heart: kil, lumen or fare-gul: cH. ehorda; c, ventricle of (he cerebrum; i>). third ventricle, ihat of the belween-braln ithalamenccphalonl ; e', fourth ventricle, that or the hind'brain and after-brain tepeneephalan and tnctcncephHiun, or medulla obloni^ta) ; ft, central canal of (he spinal cord.

show that the oral pit corresponds in position to the headend of the gut-tract. The formation of the pit is, in effect, a piishin^-in of the surface ectoderm to meet the alimentary entoderm.

A second important factor in the development of the face is the appearance of the first and second visceral arches, which occurs in the third week. As pointc<l out in a preceiling section, the flnt viBceral arch divides into the mandibular areh and the maxillary process (Fig. 62), the latter being the smaller and appearing to spring from the mandibular areh. Both the maxillary processes and the mandibular arehes grow toward the mc<lian lino of the ventral surface of the body. Owing to the growth of these struct

itren and to the sharp flexion of the head and neck that MM!iirH between the twenty-first and the twenty-third day, the (inil pit becomes very much deeper and acquires more definite boundarie.s. During the third week it is a iossa of |H<nUif|^oul outline. Its upper boundary is ibrmed by the unpaired nasobontal or nasal process (Fig. 67, A), which is BAHentiully it thickening on the ventral wall of the forebrain vcHiele, brought int<> close relalion with the to^ea by the flexion almve reierredto. The lower boundary is formed by the tnundibular arches, while the lateral extent of the fosea 'i» limited liy the maxillary process of eaeh side.

KiHin atWr the appearance of the oml pit, the future nnre.s are fiifeHhadowed by tile development of the two oUactorr plates, flititiited one ou each side of the nasofrontal process, widely iH'iHinited from each other. These epithelial areas, which (HHin beeome dejiressions, the nasal pits, are closely united witli llie wall of the fore-brain vesicle from the first; they develop mibitefpiently into that [«irt of the nasal mucous inemlii'Utie which Is concerned esjiecially wilh the sense of Bmell. Thi.H fact Iwcomes very significant when it is rememberttd that the olfaotory bulbs, with which the olfaetorj- epithelium aNsumes intimate relationship, are outgrowths from the bniiii.

The nasofrontal process, during the fifth week, becomes mui-h ihii'ki'tu'd idoug ils lateral margins, forming thus the Clobular pTOCosses (Fig. 07.^1), which constitute the inner l)ouiKlnrii!.H of the nasal pits. At the same time, there grow downward and forwanl from the nasofrontal process two ridges, one on each t<tde, the lateral frontal processes, which form the out«r Imundaries of the nasal pits (Fig. (i7, A). In this manner the pitd become mmh increased in depth. The lateral frontal process projects between the nasal pit and the maxillary process, its line of contact wilh the latter strui'tiirc being marked by a groove, the uaso-optic fttirow or lacrimal groove. This groove later completely disappears; it i;t of imixirtiuiee, however, as indicating the position of the now developing nasal duct, which will l)e referred to hereafter. The nasal pit-* are widely in commnniciition wilh the cavity of the primitive mouth. About the fortieth day, however, the extremities of the maxillary processes have grown so far towani the median line that tlicy have met

Tie. ei — DeTeloproent of the Cuw ot the bummn cmbirn lllie) : A, emtiDo of ■bout twenly-nine dsyi, Tbe iiuaarrontai plalu difltrvuliBllnH Into pi-cicoiBUii BlobDlana, tow md which tlio maxillary proeenBea of Ural viscenil mreh are cxlendiDg-. B. embiyo of atioul thiny-(>iur ilayB : the glubular. lal«nil fKinUl, and mailllai; processuK are In apposition : thu primitive npcnln; a now belter detlned. C, embryo of about thu elvblh week. Immediate bnundarlei of moutb are more definite and the now.) iirincesnre partly (brmed. external ear appearing. D. embryo at end of second month.

and united with the lateral frontal processes and with the nasofrontal process (Fig. 67, B and C). In this manner the nasal pits become separated from the oral fossa, each of these openings aeqiiirinfi more definite lionndaries. It is apparent from this description that the upper boundary of the primitive oral cavity is not identical with that of the adult mouth. The nasofrontal process is the forerunner of the intermaxillary portion of the upper jaw, including the corresponding part of the upper lip and of the nasal septum and bridge of the nose.^ The lateral frontal process becomes the wing of the nose. By the completion of the changes here noted the face aajuires more distinctive form. It will be seen that the upper jaw proper results from the metamorphosis of the maxillary processes. The manner in which its sinus, the antrum of Highmore, is added, as well as the ossification of the jaw, will be considered hereafter.

The development of the eye will be described in connection with that of the sense-organs. In so far as the eyes have relation to the external form of the face, it will be sufficient to say that the surface ectoderm is iuvaginated in the fourth week to form the lens-vesicle, this sac, which gives rise to the crystalline lens, being covered by two little folds of iVtcKlerm, the primitive eyelids; that the organ is situated on the siilo of the head, in marked contrast to its position in tho matun* state ; and that the naso-optic ftirrow, previously ivK^rnnl to» jmssrs from the inner angle of the eye toward iW winvr ^*f the nosi*. The development of Ihe face having Uv« ivMutinl t>ut in a giMieral way, the individual parts may tv vXHwivlonxl j^»|Kirately.

THE MOUTH.

tV vx'x ik^w hrlotly, for the sake of convenience and clear*»%^s I'V s\<vli\^r history of the development of the mouth, Hv 'UkI »{io tiiM stop to Ih» the appearance, at the twelfth ^b\;k »4 t^\^ ^^ y4»W« By the enlargement of the anterior . .ui '4 ilv iKinnt tuU* to torm the cerebral vesicles, and bv

the development of the visceral arches^ this area becomes a depression, the oral pit. The pit is at first bounded caudad by the cardiac prominence and cephalad by the fore-brain vesicle (Fig. 57). In the third week the oral pit becomes a five-sided fossa, owing to the growth of several new structures. These are the unpaired nasofrontal process, which bounds the fossa above, the mandibnlar arches, which bound it below, and the maxillary processes, which form the lateral boundaries (Fig. 67). The mandibular arches do not actually unite with each other until the thirtv-fifth dav. A transverse groove appears on the outer surface of the united mandibular process, the elevation in front of which is the lip ridge, while behind the groove is the chin ridge; these ridges respectively produce the lower lip and the chin. The angle between the maxillary process and the mandibular arch corresponds to the angle of tli(» future mouth. In the sixth week — about the fortieth day — the oral fossa acquires a new upper boundary, whi(!h separates it from the nasal pits, by the growth of the maxillary and lateral nasal processes The primitive oral cavity, as before mentioned, is at first separated from the gut-tract by the pharyngeal membrane (Fig. 6G). This structure ruptures at some time during the fourth week, thus bringing the mouth into communication with the upper end of the gut-tract. The exact location of the pharyngeal membrane with reference to the adult pharynx is somewhat difficult to define; it is certain, however, that the primitive mouth includes more than the limits of the adult oral cavity, comprising, in addition to the latter, the anterior part of the adult pharynx. Reference to a sagittal section, as in Fig. 66, shows the relation of the oropharyngeal cavity to the brain-case ; in the tissue separating the two the floor of the cranium is subsequently formed. A little evagination from a point (Ap, Fig. 66) in the back part of the primitive oral cavity becomes the anterior portion of the pituitary body or hsrpophysis, the posterior lobe of which develops as an evagination from the floor of the primary fore-brain vesicle. With the development of the floor of the cranium, the hypophysis becomes entirely isolated from the oral cavity. A little pouch or recess usually demonstrable in the median line of the roof of the pharynx of the child, though not always present in the adult, is the persistent pharyngeal end of the diverticulum that forms the hypophysis; it is known as the pharyngeal bursa or Bathke's pocket.

Very soon after the formation of the upper jaw in the nuinnor above described, the oral surface of the jaw presents two parallel ridges. Of these, the outer, which is the larger, doV('h)pH into the upper lip, while the inner smaller ridge betH>iU(»H the gum. The lip and gum of the lower jaw are produiHHl Hiniihirly, at the same time or a little later. So far, tho only d^Mnarcation between the mouth and the nasal levity in furniHhed by the tissue representing the united linmthHMital, hitenil nasal, and maxillary processes, the nares ii|H'hihft widoly into the cavity of the mouth posterior to this

I^Mlttloh,

'V\\\> rorniation of the palate, however, effects a separation Ih^Iw^h^U thr two that gives to each space its permanent limita\\\^\\^y i>u ihr inner or oral surface of the upper jaw two «ih\'iritko phJiH^tiouH appear, one on each side, which are \\s\^ y\y\\\\\\\'\\{n of the Aiture palate. These gradually grow V\»n\«U'aI iHioh othrr, the tongue, which has meanwhile been slv\\^l\»|»iu)^, p»H»hvting upward between them. In the eighth VUH Ki Mhiou v»l* \\w^K' two lateral halves of the palate begins ^l' \\w^\ wwWvwA' <»\li*«»inities. By the ninth week union has Uks^u I'lmn* a** lar back as the extent of the future hard ^H^iU^s mid. I\N llio t^h'venth week, the constituent halves of ihiv joll \u\\\\W have uuiteil also. As these two halves ap^»u^^v^K \H\K'\\ \»tlirr the tougu<» recedes from between them, v»\\ii4j^ lo \\\\^ v(iH»wth t»r the lower jaw, so that, when union imui ', thai oi^au iHHMipies its normal position under the ^^^I»U^. ( Vv.Hum t\irmation within the soft tissue first formed puulurr-' \\w }mlute piHuH^sses of the superior maxillae and of tl*o juiluio Imiiert, wliioh prtH*esses collectively constitute the hard palutr of the adult. The intermaxillary bones are toiiiu'd wiihiu the primitive |mrtiti(m between the mouth and the uau*.-.. The rompletiou of the palate definitely marks otl tho uasil t'tiambers fixuu the mouth, thus dividing the early oral luivity into a lower sjmkh*, the true mouth, and an upper region, which is essentially a part of the respiratory system.

The uvula appears during the latter half of the third month as a small protuberance on the posterior edge of the soft palate.^

The Teeth. — The teeth, morphologically considered, are calcified papillae of the skin, capped by a layer of peculiarly modified and calcified cells of the epidermis. Although in man and the higher mammals the teeth are found only upon the gums, in certain lower types they have a much wider distribution, occurring upon the roof and floor of the mouth and in the pharynx, and also, in selachians, upon the general skin-surface, in which latter case they are so modified as to constitute scales.

The dentine and cementnm of the tooth, as well as its pnlp» are derived from the mesoderm ; the enamel is a direct derivative of the overlying ectodermic epithelium. Mammals are said to be diphyodonty since they develop two sets of teeth ; while such groups as sharks, which continue to produce and lose new teeth throughout life, are denominated polyphyodont

The development of the teeth is inaugurated in the sixth week of embryonic life by the multiplication of the epidermal cells covering the surface of the gums to form a linear ridge. The growth of the ridge is away from the surface, so that the new structure projects into the underlying mesoderm. This horseshoe-shaped ridge, which corresponds in direction and extent to the line of the gum, subdivides into two parallel ridges, of which the outer marks the position of the future groove between the gum and the lip ; the inner is the dental shelf or dental ridge, which must be regarded as the earliest indication of the future teeth. The dental shelf extends into the underlying mesodermic tissue, not directly

• Deficiency of union of the halves of the palate, resulting in a median

fiuBure, constitutes the deformity, cleft palate. This deficiency may be limited to the hard or to the soil palate, or it may affect both, or it may be seen in the uvnla, either alone — deft or bifid uvula— or in conjunction with cleft palate.

iliiwiiwiinl l>iit in an oblique direction toward tbe inner op llitKiitil wirfnoi- of the gum. While the dental slielf is growliiKt it" liiK' of L^ounectiuD with the surface ectoderm is iiiui'ktKl liy tiw superficial dental groove, wliich at one time wtifl hHiktKl iiiKiii as being thu first evidence of tooth-formation.

lI|Min lh« itidd of the dental shelf opposite the free or oitil Niirfiici), individual protuberances develop, corresponding ill niuiilHtr to that of the teeth of the temporary set — ten for riioh Jiiw. ICach little projection consists of a mass of ectoili)i'inli^ I'clU. which soon becomes expanded at its deep exIroinity, iMTomiri)^ thus club-shaped and later Ha-sk-tihajied, anil which in <ni]led llie enamel-sac or primitive enamel-Kenn, nUlcv tJio eiitiiucl of the tooth is developed from it (fig. <JS),

Mcntiwhile the continnity of the original dental shelf is broken by the di»ip|K-uranoc of the t*Il3 in the intervals between tlio in<)iviilual ennmel-germs, each germ iiccoming thereby isiilnti-d from itn neighbors. The neck of the flask shaped enamel-germ becomes reduced to a slender strand of cells and finally disappears, so that there is no longer any connection between the enamel-sac and the ectodermic cells of the free surface of the gum. While the enamel-sacs for the temporary teeth are growing in this manner, the corresponding structures for the teeth of the permanent dentition bud from the inner side of the dental shelf — that is, the side looking toward the tongue — except those for the three permanent molars, w hich grow backward toward the articulation of the jaw from the position of the second temporary molar.

As the enamel-germs grow downward into the mesodermic tissue, the latter sends up a number of conical projections, the dental papills, one for each enamel-organ. This dental papilla, of mesodermic origin, is the parent of the dentine and of the pulp of the tooth. When the dental papilla and the enamel-sac meet, the sac becomes invaginated, its under surface assuming a concave form. The enamel-sac at this stage therefore is a double-walled cup which caps the dental papilla. It is at about this time thatthe connection of the enamel-organ with the surface ectoderm is lost.

The further evolution of the enamel-organ consists essentially in the arrangement of its constituent cells into three layers and the formation, by the deepest of these three layers, of the special elements of the fnlly-developed enamel — the enamel-prisms. The most superficial stratum of the enamelorgan is composed of low columnar or polyhedral cells; the deepest layer, that nearest the papilla, the so-called membrana adamantina, consists of beautifully regular columnar cells, the ameloblasts or adamantoblasts ; between the two is a group of less characteristic epithelial elements. The cells of the deep layer, the enamel-cells, are alone con(?erned in the production of the enamel. The enamel-organ for a time covers the entire dental papilla. During the course of development, however, the growth of that part of it covering the future root of the tooth aborts, leaving the crown alone covered with the enamel.

The first step in the formation of the enamel-prisms by the enamel-cells is that the protoplasm of the deep extremity of each cell becnmex homogeneoii!>, and a tuft develops on the end of the pell, projecting loward the papilla. By the calcifieatiuD uf this tuft the formation of an enamel-prism is l)Cgun (Fig. 69). The process of caU'J Heat ion continues to advance from the deep or papillary aspect of the enamel-org-aii toward the surface. From this it comes about thatthe newest enamel is next to the enaniel-celis, or, in other words, nearest the surface, and also that the cnamel-prisma are arranged in a directiun generally vertical to the free surface cf the tooih. The formation of the enamel of the milk-teeth begins in rii. M -Himii-rtiiiKniniiuaik (Is- the latter part of tUe/uurih monlh. iMieiiylng B.iiiiii«i.«rgnii (Tuur- The middle layer of the enamelii i,..i.iit™i™ii.r.ren«moi- Q^^p becomes greatly altered in

mil; J mill K.oi'Umf Inner larer ^ , , ° ■'

conatttution, owing to the accumulation of fluid and to the reduction of its colls to the form of ihin plates, the appearance being rather that of eonii«;tive IImiic than of an rpithelial structure. The sujierlicial layer of w\U uiidi'rf{iii-M atrophy, their exact fate not being known. The ritii'ithic ii>uinaut of the onaniel-organ is found upon the IVee nuifac-c of (he tooth for a variable time after its (iniplion, I'oimtilultMg (he membrane of Naamyth.

'I'lie dintaJ papilla hax beeu relVrrcd to as the structure lliat K<veH rise to the dentine. It origitintes from active milltiplicatiiiii of llie mesiMlermii- cells. The number of |MpillH> cnrrcrtpondH to the number of enamel-organ^;. As the [itipillu grows toward the enamel -organ it early acquires vawnlurlty. The shu]ie of the papilla, whether that of an ineinor, of a L^nine, or of a molar tooth, is determined by the fthiipc which the eniiniel-orgiHi assnnics. the conneetive tissue cells upon the surface of the papilla assume distinctive character, becoming large and branched, and constitute the so-called odontoblasts (Fig. 69). They are virtually modified osteoblasts. Forming a continuous layer, they have been styled the membrana eboris. Between this layer of odontoblasts and the enamel-organ a layer of intercellular substance appears, the membrana prsformatiya. The odontoblasts now send out processes toward the enamel-organ, which are known as the dental processes. Calcification begins upon the surface of the papilla and progresses toward its center, but is not complete. Small uncalcified areas, corresponding to the globular spaces of the completed tooth, remain next the enamel. The dental processes likewise fail to become calcified, and these are the adult dentinal fibers occupying the dentinal tubules of the finished dentine. The odontoblasts continue the formation of dentine until the dental papilla is entirely surrounded by it. What remains of the papilla, upon the completion of the tooth, constitutes the pnlp, a highly vascular connective-tissue substance supporting upon its surface the odontoblasts. The deposition of dentine begins in the latter part of the fourth month.

During the metamorphosis of the dental papilla the mesodermic tissue immediately surrounding it undergoes slight condensation to form the follicle of the develoj)ing tooth. As the enamel-organ recedes from the surface, the follicle increases in extent to such a degree as to envelop the entire rudimentary tooth. Only that part of the follicle which covers the future root of the tooth is of subsequent importance, however ; undergoing partial transformation into true bony tissue, it gives rise to the cementom or crosta petrosa, while the unossified external fibrous layer constitutes the lining periosteum of the alveolus (Fig. 68).

The development of the permanent teeth is precisely analogous to that of the milk-teeth. The enamel-germs for the permanent teeth, with the exception of the molars, bud from the lingual side of the dental shelf in the seventeenth week (Fig. 70), the germ for the first permanent molar appearing about a week earlier at the posterior extremity of the dental shelf after llie manner of a milk-tooth, The germ for the second molar XhvXa fr^ini the tieck of the first molar in the tliir<l month after Itirth, while that of the third molar, the wisdom tooth, springs from the neck of the second about the third year. At birth, therefore, the gums contain the

enua at o inl1k-t>i'>Ili and ul

, llilckenetl oral eplLLeUum: «i, i •rown out at u rrom the dicIe (>) of II. Uecki-raciLrtlligt^.

two sets of teetli except the second and third permanent molars.

The eruption of the temporarr teeth begins usually at about five and a half nioiilhs after birth wilh the appearance of the central incisors, and is coniidete at from eighteen to thirtysix months, when thu sccniul molars are cut. The first t«eth of the penuftnent dentition are the tirst mohii's, which are eru)ited at ahoiil the sixth year. The accompanying table shown the time and the onlir of eruption of the teeth :

THE MOUTH. 143

Temporary Dentition.

Central incisors 5} to 7 months.

Lateral incisors 7 to 10 months.

First molars 12 to 14 months.

Canines 14 to 20 months.

Second molars 18 to 36 montha

Permanent Dentition.

First molars 6th year.

Central incisors 7th year.

Lateral incisors 8th year.

First premolars 9th year.

Second premolars 10th year.

Canines 11th to 12th year.

Second molars 12th to 13th year.

Third molars (wisdom teeth) 17th to 21st year.

The Salivary Glands. — The salivary glands, which in mammals consist of three pairs, the parotid, the submaxillary, and the sublingual, develop as outgrowths of epithelium from the lining mucous membrane of the mouth. The epithelial elements of the glands are therefore of ectodermic origin. The growth of the submaxillary gland begins in the sixth week, that of the parotid in the eighth week. Each epithelial outgrowth is at first a solid cylinder, which undergoes repeated branching and acquires a connective-tissue framework and capsule from the surrounding mesoderm. It is not until the middle of the fifth month that the lumen of the gland appears. This is brought about by the moving apart of the epithelial cells composing the cylinders and their branches. The main duct of the gland first becomes hollow, then its branches, and finally the lumina of the alveoli make their appearance. The respective sites from which the several glands grow correspond in a general way to the |X)sitions at which the ducts of the adult glands open into the mouth-cavity.

The Tongfue* — Although the tongue originates from tissues belonging really to the walls of the pharynx, its development may be conveniently considered in connection with that of the mouth because of its relations in the mature organism. This organ, composed chiefly of muscular substance, is formed from three originally separate parts, an anterior unpaired fundament, and two posterior bilaterally eymmetrical segments. The line of union of these three parts is indicated approximately in the adult organ by the V-shaped row of circumvallate [Htpilla; on the dorsum of the tungtie. The anterior part of the tongue develops from a small unpaired tubercle, tlio tu1)erculiim impar, which grows from j the median line of the floor or anterior wall of the pharynx between the first, or mandibular, and the second, or hyoid, areh {Fig. 71, 6). the posterior segment of the tongue results

., mm. (nbout 25 lo 30 linja). /, II, III, 1 , , uumt ,»oeral lurrow*; v, «inu» pr»pervi«all«, comprtaliig third and Fourth outer furrows: l,S,3, 1., vlieeral archei, s>ch with lU v<8(vml-Arch vc«el; e, tubercalum Imparl 7, orlflce of larynx : S, pulmunary evaKiDalian.

from thegrowing together of two lateral halves, which develop from the anterolatend walls of the pharynx at the position of the second and third visceral arches, but not from the i arches themselvos. These ridges arc sometimes described aa ' the fused anterior (ventral) extremities of the arches just mentioned. The unpaired tubercle increases in size to such an extent as to constitute the major part of the organ. In the median line of the anterior wall of the pharynx, immediately behind the tuberculum impar, the epithelial lining of this cavity pouches forward and downward to develop later into the middle lobe of the thyr()id body. As the ridges which are to form the posterior [wrt of the tongue lie laterally and posteriorly to this median evagination, they completely enclose it in the process of fusing with each other and with the anterior tubercle. In this manner a canal or duct is formed leading from the surface of the tongue at the angle of junction of its three segments down to the middle lobe of the thyroid body, the latter meanwhile having descended from its original position. This canal is the thyroglossal duct or canal of His. During the further progress of development, the canal suffers obliteration, its only vestige beiug the orifice, which is known as the foramen csecum of adult anatomy.

The papillae of the tongue are found exclusively on the part derived from the tuberculum impar ; the line of union between the anterior and posterior parts lies therefore behind the row of circumvallatc papilla?. The papillae begin to make their appearance as early as the beginning of the third month.

Prior to the union of the two lateral halves of the hard palate, by which the primitive oral cavity is divided into the mouth proper and the nasal chambers, the tongue projects upward between the palate-shelves, almost completely filling the primitive mouth. As the palate-shelves approach each other, however, the tongue gradually recedes to its subsequent normal position.

THE DEVELOPMENT OF THE NOSE.

The nose being an organ of special sense, its development is described in connection with that of the other special-sense organs in Chapter XVI. Owing, however, to its important relation to the other parts of the face, it is desirable to refer to its evolution in this connection. For a more detailed account, the reader is referred to Chapter XVI.

The first indication of the organ of smell is in the form of the two patches of thickened ectoderm, the nasal areas or olfactory plates, which appear on the head ward side of the oral fossa in the third week of development. At the end of the fourtli week the areas are depressed and constitute the nasal pits (Fig. 67, A). The nasofrontal process, a mass of thickened mesodermic tissue, lies between them. During the fifth week the lateral edges of this process become thick and rounded, forming the two globular processes, while growing outward and downward from the sides of its base are the two lateral nasal or lateral frontal processes. Thus the nasal pits, which corresiK)nd with the position of the future anterior narcs, become bordered on the mesial side by the globular processes and on the outer side by the lateral nasal processes. Below, the pits arc continuous with the oral fossa. Owing to the continued growth of these masses the pits gradually become deeper. The lateral nasal process is separated externally from the maxillary process of the first visceral arch by a groove, the naso-optic fturow. The lower extremities of the maxillary and lateral nasal processes soon unite with each other and advance toward the median line below the nasal pit. In the latter part of the sixth week they unite with the nasofrontal ])rocess and thus separate the nasal pits from the oral fossa and furnish the basis of the up{>er lip. The nasal pits are now the anterior nares, and the nose is represented by the irregular masses of tissue surrounding them. While the orifices of the nares are separated from the orifice of the primitive oral equity, their deeper parts are continuous with the latter, there being as yet no hard or soft palate.

In the eighth week the nose first acquires definite form, owing to the continued growth of the masses of tissue referred to above. The nasofrontal process forms the bridge of the nose with the nasal septum, and also the intermaxillary part of the superior maxiihe and the oonnoetive-tissue parts of the upper lip. The lateral frontal process becomes the ala of the nose. The nose is still verv broad and flat in the third month, after which time it grailually assumes its characteristic form.

+++++++++++++++++++++++++

CHAPTER X.

THE DEVELOPMENT OF THE VASCULAR SYSTEM.

The vascular system, including the blood, the heart, and the blood-vessels, begins its development very early in embryonic life.

While the heart is formed within the body of the embrj-o, the blood and the earliest blood-vessels have their origin in an extra-embryonic structure, the yolk-sac. It is noteworthy that all parts of the vascular system proceed from mesodermic tissue, the heart and the vessels originating from clefts within this structure, and being lined, therefore, with endothelial cells.

In corres[)ondence with the varying relations which the embryo sustains toward the fetal ap])endages at different times, its circulatory system is distinguished successively by certain special features. Thus, during the activity of the yolk-sac as an organ of nutrition, the vitelline circulation is present; following and supplanting this is the allantoic circulation, which latter, in turn, gives place to, or, in fact, becomes the placental system of vessels.

THE VITELLINE CIRCULATION AND THE ORIGIN

OF THE BLOOD.

The seat of the first formation of the blooil- vessels and of the blood is the wall of the yolk-sac, entirely outside of the body of the embryo. The wall of the yolk-sac, the reader may be reminded, consists of the extra-embryonic splanchnopleure covered with a part of the somatopleure. The mesodermic layer of the sac exhibits — at the end of the first day in the chick — a network made up of cords of cells, the angioblast (Fig. 72). Interspersed throughout this network are groups of cells, the substance-islands, which lie within the meshes of the network in relation with the cords

of cclU composing it (Fig. 72}. Bolli llie c«Us of the cords and of the sulistauce-isbinds are meBenphvmal t-etls. The superficial cells of the cell-cords become fiatttined iu each case tu constitute a conliniions layer \?)iich eocloaes the renininiiig culls ortli^' cjLiI.and ihey thus fnrm the eiidutheliul wall of the future b1m>d*- - vessel. The cells of the sub -I stance-iiilunds move apart

^f^ '^i and acquire prolongation!) W T or processes which intercom's , 1 mnnicate, while a gektin' ^ fl ous or f«raiHuid intercellular

ftnbslanee is formed, thus ~Bi pi^hicing an embryonal connective tissue in relation with the network of developing vessels. The solid "vessels" thus formed acquire luniina — on the second day of inculution in the chick — h_v the penetration of fluid from the surrounding niesodernt, this fluid cmwding the cells a|iart, toward the vessel walls. The channels of the vessels are at fin*t quite irregular, being at some points entirely blockeil, at others merely

• «iu, B. oncrouchcil upon, by masses

of splicniidal cells in connn'Clion with (he vessel walls. arr ihe blood-iaUndB, the aggrt^itions l^nvvd (h»> fetal nd blo-Hl-eells (Fig. 72). ' ■UpuJ-bdauib multiply by mitotic divi• l«rt, suci-eesively become detached and .■•.■Ml-.stmuit. This process continues until X\m*< ci'ILs the enrthioblasts, the first corpuscular elements of the fetal blood, are at first colorless, but soon become pale yellow. Their formation goes hand in hand with the formation of new blood-vessels. Their color deepens somewhat, liemoglobin developing within the cytoplasm. Their nuclei are hirge and reticular. The majority of them acquire small dense nuclei and are then called normoblasts. The erythroblasts continue to undergo mitotic division in the bloo<l-stream just as they did in the blood-islands, division being seen in the embryo chick up to the sixth day. In man, multiplication of erythroblasts occurs quite largely in early fetal life, particularly in regions where the circulation is slow, as in the liver, the spleen, the bone-marrow, and the lymph-nodes ; while in later fetal life and after birth it takes place in the red bone-marrow only.

It is especially noteworthy that these early fetal bloodcells are nucleated in contradistinction to the adult nonnucleated red. blood-corpuscles ; and that the nucleated form is present throughout life in all vertebrates but mammals.

Up to the end of the first month the nucleated red cells are the only corpuscular elements found in the blooil. In the second month the non-nucleated red blood -disks, the erythrocytes, make their appearance, and either in the third month or very soon thereafter outnumber the nucleated cells. Diflferences of opinion obtain as to the mode of origin of the erythrocyte, but the prevailing view is that it results from the normoblast by the loss of the nucleus of the latter. The nucleus becomes globular and more dense, assuming in some cases a dumb-bell shape, and is extruded from the cell, after which it is thought to undergo partial disintegration and then absorption by leukocytes. Some observers maintain that the nuclei are dissolved within the cell. Nuclei in the process of extrusion have been observed in cat-embryos. After extrusion of the nucleus the remaining cytoplasm of the cell assumes the biconcave form of the adult red blood-corpuscle or erythrocyte.

The origin of the leukocytes is a somewhat unsettled question. They are found in the blood of chick-embryos at the eighth day and in the rabbit-embryo at the ninth day ; in the

humanembryo they are seen in the second month. It is probable that they originate in the lymph-nodes, the bonemarrow, the liver, and the spleen during fetal life, but after birth only in the bone-marrow, the lymph-nodes, and the spleen. Their birthplace would be, therefore, lymphoid tissue and their ultimate origin mesodermic. It has been suggested that they may be derived from young ery throblasts ; this is denied by Minot. Beard assigns them an entodermal origin, claiming that they are produced by the entodermal epithelium of the thymus and of the tonsil. From the investigations of Engel and of Florence Sabin it would appear that they are first seen in the blood and the lymph-nodes at the same time.

The blood-platelets have been variously interpreted as small nucleated cells and as fragments of broken-down leuk<K?yt(»s. According to the recent work of Wright they are fragments of the processes of the giant cells (myelocytes) of bone-marrow.

Limiting the first network of vessels on the surface of the yolk-sac is a circular vessel, the sinos termiiialis (Plate VI.). Since the yolk-sac is relatively so large that the body of the <'ml)ryo appeai*s to rest upon it, and since the surrounding Homatopleure is translucent, a surface view of the ovum at this stagt* shows a vascular zone encircling the embryonic urea and the later Ixnly of the embryo. This zone is the area ▼aaculosa, or vascular area, the seat of the earliest formation of bhxMl and of blood-vessels of the embryo.

Th(» blood-yeBsels originate, as shown above, from the angioblastio network of m<»senchymal cell-cords of the vascular an»a, the cords of cells, at first solid, gnulually becoming hoUowed out to form the vessels. The vascular network at first forni<»d extends by a pnK*ess of budding over the walls of the yolk-sac an<l thence along the vitelline duct into the ImkIv of tln^ embryo. The budding consists in the extension of vessel sprouts or cell-cords — probably from proliferation of the terminal cells of the vessels last forme<l — the sprouts being solid at their ends, since the excavation of a sj^rout alwavs occurs a little later than its forward extension.

THE VITELLINE CIRCULATION. 151

Neighboring sprouts communicate with each other to a greater or less extent. In a human embryo of about eighteen days the extension of the vessels — appearing macroscopically as fine red threads — along the vitelline duct is well shown. Having reached the bcxly of the embryo, the vessels take their course to wanl the primitive heart, which has meanwhile been d(;veloping. From the anterior and posterior and lateral limits of the vascular area — using these terms with reference to the axis of the embryonic body — four pairs of vitelline veins converge toward the vitelline duct and unite to form the two vitelline or omplialomesenteric veins. These veins, after entering the body of the embryo, pass headward along the wall of the intestinal tube and empty into the lower or caudal end of the primitive heart. The trunks, which are to constitute the vitelline arteries, after entering the body with the vitelline duct, pass upward along the dorsal body-wall, within the dorsal mesentery, to become continuous with large arterial trunks that have i)roceeded from the primitive heart.

The large trunks referred to are the visceral-arch vessels, which unite to form the primitive aorta?. The visceral-arch vessels (see Fig. 60) are a series of five pairs of arteries that arise by a common stem, the truncus arteriosus, from the upper end of the primitive heart. They pass along the respective visceral arches toward the dorsal surface of the bo<lv where all the vessels of one side unite into a common trunk, the primitive aorta. The two primitive aorta?, passing caudalward in the dorsal mesentery, give off, as their largest branches, the two omphalomesenteric or vitelline arteries above referred to. the development and the regression of the visceral-arch vessels correspond with the growtli and the decadence respectively of the visceral arches. Not all the vessels are present in a fully-developed condition at any one time, the first pair having begun to atrophy before the fifth pair makes its appearance. The metamorphosis into certain adult vessels of such of them as persist will be considered in a later section.

This system of vessels constitutes the vitelline circulation, the manifest function of which is to convey nutritive mate

152 TEXT-BOOK OF EMBRYOLOGY.

rial from the yolk-sac to the embryo. While the vitelline circulation i.s of great importance in any ovum provided with abundant nutritive yolk, such as that of the bird, it is of (U)mpanitively slight consequence in man and the other higher mauimals, and it must be regarded as a vestige of the avian or reptilian ancestry of the mammalian ovum, or, at Icanf, an a reminder that the mammalian ovum was originally provided with an abundant yolk. It must be borne in mind, how('Ver, that the mammalian blastodermic vesicle imbibes frniM th(! walls of the uterus a richlv nutritive albuminous Ituid, wich may be taken up later and carried to the embryo by \\\i\ vitelline circulation. This system of yolk-sac \v*^ni'\H diHuppears with the regression and disap])earance of the jolt-HHc - -in the human embryo at about the fifth week. The Inlni-i'inbryonic portion of the right vitelline artery persists, hoWdViT, to bc<!onH! the Enipeiior mesenteric artery of the adult. To riMidrr ilhi comprehension of the later phases of the Viiw'nliM' HVMdMU more simple, their consideration is deferred MMlil ihn dnvi'lopnient of the heart shall have been described.

I'lm DIJVKLOPMENT OF THE HEART.

Thn llt!lirt| when Htudied in the lower-type animals, is Minn In bn niorpliologieally a dilatcil and specialized part of a vunriiliir trunk enil>e<lded in the ventral mesentery. In miiiii thi» Hint Inndunient of the heart appeai-s at a very early

Iii!»ioi| nunu'ly, brlore the splanehnopleure has folded in to linn ihn pill t met. or. in other words, before the end of the mM'tiiid sviM'k. Tlii" Inndanu'iit, in all higher vertebrates, is bilalttntl, huviiiK thi^ lonn of two tubes prcKlucecl by vacuolation of tht) nplitnehnie ineHoderm an<l lying widely separated, iin«« in ntifh half of th(« Htill spread out splanehnopleure (Ki^. 7.1), .1). A tniiiHVi'rHe K(*etion through the future neckregion of a hiu'i'p wv nil»l»it-embryo shows tiie tubes cut auritnn, hiiMv ihrir long ii\(*s ans [mnillel with that of the binlv (Kig. 71). With tiu< li>lding in of the splanehnopleure uiid the union of tli«' (ulgi^s of its folds, the tubes are carried tiiward ea4^ii other, and hubse<|Uently, by the disappearance

THE DEVELOPMENT OF THE HEART. 153

of the tissue intervening b^etween them, their cavities become one (Fig. 73, B ami C). After the formation of the guttract, therefore, and the simultaneous apj^aranco of the ventral body-wall, the heart-fundament is a single straight mesodennic tube, situated in the pharyngeal region, in close relation with the ventral wall of the bmly, between the latter and the fore-gut. Reference to Fig. 73, C, will show that

Fio, 7S.-SehPin«il

OM se on of abb n b

70 n show dcTelnpmcnt of

heart: A. embryonfc

■ > e[> ad oHl: fl. more

kdvanced bUrc, the h[

an hnnp euro p«rt y f d d

C Ep an hnoplourc folded

In to furra gul-tra«. th

e <ancr Slrahl).

the heart-tube is separated from the bwly -cavity (or coelom) on each side by a layer of the mesoderm, and that these two layers connect the heart dorsiilly with the gut-tract and ventrally with the iMxly-wall, forming rcs|iectivoly the meso' cardltun anterioa and the mesocardiam posteriua. These folds temporarily divide the upper portion of the body-cavity into two lateral jtarts.

The disap]>earance of the stratum of mesoderm immediately surrounding the heart-tube and tbo differentiation of the tissue limiting peripherally the space thus formed, results in the production of a second larger tube enclosing the first. The cells of the outer tube become specialized

1^4

TEXT-BOOK OF EMBRYOLOGY.

iiit'i miucle-ceUs, wliicli arc to contitilutc the fiituro heartmiixcle, while those of the inner cylinder Rutteii and assume the endotheliotd type to become the endocardium. The gniwtli of c«ntmlly projecting proceBaes from the muscular wall and th« oiitpoi-kcting of the endothelial tube to cover these proceHHCB ami line the spaces enclosed by thera foreshadow thv tpongy churjctcr of the inner surface of the adult heart, with it8 colnmne camen and omsciili pectinati. It is signifiCaiit, as lihowing tlic contractility of niidiff'erDiitiated proto

plasmic cells, that the heart l)^n9 to pulsate even before the ap]>carance of any muscnlar tissue in its walls.

The upper end of the heart-tube ta(>ers away into the truncus arteriosus (Fifi. 75, 4), a vessel which bifurcates into the first jMiir of visceral-arch vessels, while its lower extremity receives llie vitelline veins above referred to. Excessive growth in length, each end of the tube Iwing more or less fixed in position, necessitates flexion or folding, the form which the heart-tul)e assumes in consequence being that of the letter S placed obliquely {Fig. 70, A). The venous

156 TEXTBOOK OF EMBRYOLOGY,

the dorsal wall of the body, with the arterial portion ventral to it, both being brought at the same time into practically one transverse plane by the headward migration of the venous, and the tailward migration of the arterial, moiety. At this time the heart is relatively so large, and the ventral body-wall covering it so thin, that the organ appears as if situated outside of the embryo's body (Fig. 62, p. 116).

Simultaneously with these alterations in position, the arterial part of the heart is being marked off from the Tenons segment by a transverse constriction, the former becoming the ventricle, the latter the auricle or atrium (Fig. 76, A). The narrow communication between the two is the anricnlar or atrioventricular canal, which soon acquires the primitive atrioventricular valves, or endocardial cushions, these being endoeanlial thickenings on the dorsal and ventral walls of the Ciinal. Growing toward cacli other, the cushions meet and unite, forming the septum intermedium (Fig. 78, B,/), which now occupies the middle of the auricular canal, leaving only its lateral portions jwtulous. The truncus arteriosus becomes delimited from the ventricle bv a circular constriction, the fretum Halleri, the proximal part of the truncus arteriosus dilating somcwliat to constitute the bulbus arteriosus. The truncus arteriosus divides into the visceral-arch vessels, as pointed out in the last section.

The Metamorphosis of the Single into the Donble

Heart. — The heart with but one ventricle an<l one auricle or atrium is found not only during the early periods of development in all air-breathing vertebrates, but is the permanent conditi<m in fishes. In the development of the individual, as in the evolution of the higher vertebrate type, the appearance of the lungs, which replace the branchiaj of fishes as an aerating apparatus, is accompanied by a division of the heart into right and left halves for the pulmonary and the general systemic circulation respectively.

The division of the human atrium begins in iho fourth week with the growth of a per|)endicular ridge from its dorsal and cephalic walls (Fig. 78, B), this being indicjited externally by a groove on the outer surface of the corresponding wall of

THE DEVELOPMKST OF THE HEART. 157

the auricle. The ridgt', gniwing downward, becomeB the septum primiun or auricular septum and fuses with the upper extremity of tiie septum intermediutii of the auricular caiiiil, thus dividiDi; the atrium into the right aiul left auricles (Fig. 77). The atrioventricnlar caual, it^ iiiiterior aud j>osterior cushions having united into the scpuim intermedium, shares in this division, becoming thereby the right and the left auriculo ventricular orifices. The division of the atrium, however, is not as yet complete ; a hiatus, the foramen ovale, exists ventral to the free ventral border of the unittnl si>ptum

tlonornlrlumcoTreipoailliieH'ith •uricuUr appcndagei r, iruneu* arterlasiu; d, ■nrlculitr canal; c, piimlUve Tcntricte. B, heart of buniBiU embryo of about tbe flllh week (Hli): <i, left auricle: b, rigbt ■aricle: r. trunctu arteiioaiu; d. IntctOTc; e. rtglit veiitrido : /. led TeDtriplc.

primum and septum intermedium. A ridge grows downward from the roof of the atrium U|H)n the right side of the septum pHmum and parallel with it; this is the septum secundum (atrial crescent) and is very much thicker than the primary septum. Its downward growth continues in such manner that it comes t<t bound the foramen ovale vontrally and below, its extremity uniting with the left extremity of the fold which later becomes the Eustachian valve, and thus forming the futuiv atmulus oralis. The psirt of the primary eeptnm which is thus jiartially eiirronnded by the free margin ' the si.'ptuiu secundum [Mjuches into the left auricle to con

158

TEXTBOOK OF EMBRYOLOGY.

stitutc a fiort of valve for the prcvculioii of regurgitation. At birtli or shortly ufler, the ventral edge of this valve-like fold unites wiih the ventral margin of the foramen ovale, thus obliterating the latter, the fohl beuoming thereby the relatively thin floor tif the fossa ovalis of the adnlt heart.'

The diTiBion of the ventride, which follows that of the auricle and wich is completed by the seventh week, is first

Fio, 78.— A, BecllOD of liBarl n SpUriiira ; 0. interaurlcular sopluni t, left auricle: f. auricular Miiitl: [

ibryo of in mm. (Hts): ci, aeiiluin c.moulh or Binm reunicnB; (/.right aurtclo; right ventricle; A, Interventricular septami t nr hamati ctnliryu of alKiiit Hie flflli week '

itidicate<l by a vertieat groove, the snlcns isterrentricalaris, seen on both thedor!*!il uml the ventnil siiHiiee tl'ig. 77), From the internal surface, corresponding to the position of the sulcus, a median centrally projecting ridge appears aud devehipg into a septum (Figs. 7K, /i, and 79, t*), whieli, however, i.s ineompkle above and in front. The deficiency thus left, the ostdiuu interrentriculaie, is obliterated by the tlowngrowth of the aortic septum (Fig. 79, k), upon the coni])letion ' Occasioiiully the foramen ovnla remainit p«tuli)iiit for iicverul weeka ur months nfter birtli or even thnniKhiHit life. Ab thin pondilion ullowx llie venous bliHxl lu minifle willi the arterial, the miKiicu of the Ixily U bluish or rynnntir, uii<) a chliil thus iiltedeil U om.\ I<> )« n - hiii<> baby."

THE DEVELOPMENT OF THE HEART 159

of which the original single ventricle is divided into the right and the left ventricles. While the interventricular septum of the completed heart is, for the most part, muscular, that portion of it which is produced by the aortic septum always remains membranous, constituting the pars membranacea septi of the adult heart. If this septum is incomplete, as happens occasionally, there is an abnormal communication between the two ventricles.

The tnmcus arteriosus, after having become somewhat flattened, is divided by the growth of a vertical septum, or partition (Fig. 79, «), into the aorta and the pulmonary artery. The growth of the partition is initiated by the appearance of two ridges on the opposite walls of the truncus, the ridges growing toward each other and finally uniting to form the aortic septum. Two longitudinal grooves which appear upon the surface of the truncus, following the growth of the ridges and corresponding in position with them, indicate the division of the vessel into the aorta and the pulmonary artery. The septum grows downward to meet and unite with the ventricular sej)tum, as indicated above. Though the three septa referred to develop indejwndently of each other there is such correspondence between them, as to position, that the effect is as if they constituted one continuous structure.

Before the divisicm of the atrium into the auricles, its walls pouch out on each side to form the auricular appendages, one of which belongs to each future auricle (Fig. 77). While it is still a straight tube, the heart receives at its venous extremity the two vitelline veins. Subsequently this particular part of the atrium is distinguished as the sinus venosus or sinus reuniens, this being a short thick trunk into which empty, in addition to the vitelline veins, the ducts of Cuvier and the umbilical veins. The mouth of the sinus venosus is guarded by a valve composinl of two leaflets. The right leaflet or fold is continuous above with a ridge on the roof of the atrium, the septum spurium (Fig. 78, a). In the division of the atrium the sinus venosus falls to the right auricle, while emptying into the left auricle is the single pulmonary vein, which is formed by the union of the four pul

AradI W«« il^ •nd^si kaslk n— » nab

• f + f

IMJWM rrom iIm- fnjiH iif llie superior vena ca\-a to the (root Iff llie infrrinr vciui cava.

The U-ft Irafict of iho valve at the month of the tdaiis vcn'iMia hiH-imnii iHrophie, as does also the septuni spiirium ; tl)'- richt ilrviiW into two partfl, one of which becomes the EaitAchUn Yalr« iit the orifice of the inferior veoa ca\'a,

THE DEVELOPMENT OF THE HEART. 161

while the other forms the valve of Thebesius, or the coronary valve, at the opening of the coronary sinus (the latter being the persistent lower end of the left duct of Cuvier). The Eustachian valve serves to direct the blood from the inferior cava through the foramen ovale so long as that aperture is present. The single pulmonary vein is in like manner incorporated in the wall of the left auricle, the four pulmonary veins in consequence acquiring separate oj^enings into that cavitv.

The Valves of the Heart. — Before the division of the atrium and the ventricle into right and left halves, the atrioventricular canal has the form of a transverse fissure, each lip of which is thickened into a ridge (Fig. 79, ^4). These ridges or endocanlial cushions are the primitive valves. When the atrial partition grows down and the ventricular septum grows up, their free edges meet and unite with the ridges, each ridge being thereby divided, on its atrial surface by the atrial or interauricular septum, and on its ventricular aspect by the ventricular septum, into a right and a left half (Fig. 79, B). Since the ridges, at their points of union with the septa, fuse likewise with each other, the original orifice is bisected into the right and left auricnloventricular apertures, the only valves of which are the ridges or cushions in question.

To trace the further development of the fully formed valves, it will be necessary to consider the changes which now take place in the walls of the heart. It has been seen that the inner surface of the heart accjuires a spongy or trabecular structure at a very early stage by the inward projection of nmscular processes from the outer tube and the pouching out of the inner endothelial tube to cover these. The wall of the ventricle in consequence is relatively very thick and is made up largely of a network of fleshy columns, the spaces of which network are lined with the endocardium (Fig. 80, A). While the outer stratum of the ventricular wall now becomes more compact by the thickening of the trabeculae — and, to some extent, l)y their coalescence — the

trabeculae in the vicinity of the atrioventricular valves dill

162

TEXT-BOOK OF E.MBJiVOLOOr.

minisii iii thickness and lose their iiniacnlar cliaracter, Iwing replaced by thin cutiucctive-tissiie cords (Fig. 80, B). That port iif the vetitridilLir ividl which surround,* the iitrioventriciilur oriliee and to wlnt-h the onducardial enshions or

primitive valves are attached, likewise becomes deprived of ninscle-cells, the remaining connective ti^Bue assuming the form of thin plates. Thc.^TC plates, with the former endocardial cushions attaehed to their edges, constitnte the

Via. tU.-OdieniD ■honing diviBlon ul Into a'lfW and pnlmnniitT Brtery wUh thi lateral leaQcla divldini; rtmpectlvely Inlo

permanent auricnlorentricalar valve-leafleta. The strands of connective tissue mentioned above as remaining after the degeneration of certain of the muacle-traheculffl arc the chords tendineae of the adult heart. Attached at one end

ALLANTOIC ANJ) PLACENTAL CIRCULATION. 163

to the valve-leaflets, their other extremity is continuous with trabeculae that have remained muscular, the adult musculi papillares.

The Bemilunar valves of the aorta and pulmonary artery appear when the truncus arteriosus divides to form those vessels. The orifice of the truncus arteriosus is provided with a valve having four leaflets (Fig. 81, A). By the division of this vessel into the pulmonary artery and the aorta (Fig. 81, £ and C), the lateral leaflets are bisected, the anterior half of each, with the anterior leaflet, going to the anterior vessel — the pulmonary' artery — while each posterior or dorsal half, with the dorsal leaflet, falls within the orifice of the aorta. The resulting disposition of the segments of the aortic and pulmonary valves is such that, in the aorta, two leaflets are situated anteriorly and one posteriorly, while in the case of the pulmonary artery these conditions are reversed (Fig. 81, C). In the fully developed heart, however, it is found that the aorta has two posterior leaflets and one anterior, and that the pulmonary artery presents one posterior and two anterior segments. In the division of the truncus arteriosus, the anterior half, or the pulmonary artery, falls to the right ventricle, and the posterior trunk, the aorta, to the left ventricle, the two ventricles lying side by side. In order, therefore, that the ventricles may accjuire the relative positions which they hold in the adult there must be such a rotation that the left ventricle comes to lie behind the right. This rotation of the heart from right to left necessarily alters the relation of the pulmonary artery, causing it to lie not directly in front of the aorta, but in front and to the left. If one conceives of a rotation of the two vessels from right to left through an arc of 60 degrees around a vertical axis, the altered relation of the pulmonary and aortic leaflets becomes at once intelligible (Fig. 81, C'and I)),

THE ALLANTOIC AND THE PLACENTAL CIRCULATION.

The development of the allantois and its accompanying system of blood-vessels is simultaneous with the decline of the yolk-sac and the vitelline circulation. Since the allan

164 TEXT-HOOK OF EMHUYOLOCY.

tois is an e vagi nut ion from the giit-trart (see p. 89), it is a Bplanchiipleuric sac, its walls consisting therefore of an entodermic and a mesodermic layer. Blood-vessels develop within the mesodermic stratum as extensions or branchea of previously existing intra-embryonic tninks. These vessels are the aUantoic arteries and veins. The two allantoic arteries are branches of the primitive aorta and leave the body of the fetus, in compiuiy with the neck of the allantois, at the nmbilicus. Having reached the peripheral jiart of the allantois, they break np into a capillary plexus, the extension of which into the villons processes of the false amnion completes the union of that strncture with the allantois to form the trne chorion (Plate III.),

the two allantoic Teina develop prxri passu with the arteries and con\ey the blood from the chorion to the fetus. Entering the body of the fetus through the still lai^ umbilieiil aperture, they find their way along the intestinal tube tn the septum transveisom — which structure may be regarded as the jiriniitivc iliaphrngm — to the region of the heart, where they <ii>en into the ducts of Cuvier. Each duct of Cuvier (Fig. 84, A) is Ibrmed by the union of the primitiye jugular vein with tbe cardinal vein of its own eidc, the cardinal and the jugular veins returning the blimd respectively from the lower and upper [wirts of the trunk. Tliis system of blood-vessels constitutes the allantoic circulation ; it is of great importance in any ovum that is developed outside of the body of the mother, as in the case of birds, reptiles, and fishes, in which classes the allantois is the organ of nutrition from the time that the yolk-sac ceases lo ]>erform that function until development is complete. In man, however, aa in all other mammals except the monotremes and marsupials, the aliant^iic circulation may be looked upon as, in a measure, rudimentiry, since it serves to convey nutriment from the chorion to the fetus only until the formation of the placenta.

The placental system of blood-yessels, apf>cHring in the thiol month willi the ilcvclopment of the placenta, includes th(^ principal trunks of the former allantoic system, the allimtoic arteries and veins having Iwcimie the umbilical vessels. The two umbilical arteries convey impure blood from the fetus to the placenta, where it circulates through the capillaries of that organ and receives oxygen and nutriment from the blood of the mother. As before stated, there is no intermingling of the fetal and the maternal blood, the two currents being separated by the very thin walls of the capillaries, through which osmosis occurs. The purified blood returns to the fetus through the umbilical veins and reaches the right auricle through the inferior vena cava, a portion of it having passed through the liver. The two umbilical veins which are present for a time fuse subsequently to form a single vein. The complicated details of the arterial and the venous trunks, and the relation of the latter to the development of the liver and its special system of vessels, may be advantageously considered in separate sections.

THE FETAL ARTERIAL SYSTEM.

The truncus arteriosus, the large artery which arises from the, as yet, undivided ventricle of the heart, bifurcates into two trunks, the first pair of visceral-arch vessels (Fig. 82, 4). These first visceral-arch vessels, also sometimes called the first aortic arches, run from the ventral surface of the body along the first visceral arches, toward the dorsum, where they curve downward and pass caudalward, one on each side of the median line, in front of the primitive vertebral column. Very soon there arise from the truncus arteriosus below the point of origin of the first vessels, four^ additional pairs of visceral-arch vessels, which similarly pass dorsad along the corresponding visceral arches, and which unite with the dorsal part of the first pair to form the primitive aorta of each side. Each primitive aorta results, therefore, from the confluence of all the visceral-arch vessels of its own side (Fig. 82). The two aortte afterward become merged into a single trunk. At first the principal branches of the aorta are the vitelline arteries. As these latter vessels become

^ It is sonietinies stated that there are six viscenil-arch vessels, the fifth of which disappears, so that the vessel here designated the fifth would rei>resent the sixth-arch vessel of the earlv condition as well as of lower forms.

in«inspicuou8, the allantoic or umbiliciil arteries come into prominence as the chief branches. Iinleod, thp iimhilical arteries may be said to be the coiitiniintion of the aorta, since the largest part of the hlooH-stream is ilivcrt«l into them. The nurta profxr continues in the median line as the caudal aorta, which latter is represented in the adult by the middle sacral artery. A branch from the fiftli arch goes to the lungs. So I'lir till' arterial f-ystem of the fetu.s presents an absolutely symmetrical armngenient (Fig. 82). Changes very soon occur, however, which lead to the asymmetrical conditioD

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found in the adult. These changes are due to the atrophy of sonic trunks and the preponderance of others. From the point where the dorsal extremity of the fourth arch joina the fifth, a branch pa.sses to the rudimentary arm (Fig. 83). The first and second arches, except their ventral and dorsal limb.s. undergo ati-opliy. The ventral limbs of the first and eecond arches persist and become the external carotid artery, while their dorsal extremities, with ttic tliird viscend-arch vessel, become tlic Internal carotid artery. The vcnlnil stem of the third arch coic^titiitcs the common carotid. The right

fonrtli-arch vessel hcroTncs tlic liglit BubclaTian, its stre&ni of Iilowl hc'uv;; .■onvcycd in the arm Ity ttio hrancli which Ims takcu its origin from the [xniit of junction of thi; dorsal t'DtU of the fourth and fifth arches. This latter branch is therefore the continuation of the subclavian. The ventral segment of the right fourth arcli would be represented in the

The foorth arch of the left • into tile thorax, itbe

1 adult by the innominate artery. B assuraea a lower position ; I comes the arch of the aorta. Sinee therightflftharclibecomes I atrophic beyond the point of origin of the right pulmonary [ artery, the dorsil end of the right fourth-arch vessel — the [ future right subclavian artery — loses its connection with the k primitive aorta, and the latter now a]>)>ears as the continuaF tion of the left fourth arch. The ventral stem of the left L third arch, which becomes the future left common carotid, Mi;d

alwo the left subclavian, ivhicti arises from the posterior or tiomil onil of thi' left fmirth ardi, nn? noM- branches of the arcti of llie nwlu. \\\wn llic tniiicus arteriosus becomes divkleil into the aoria anil the piilmnniiry artery, the left fiftharch vessel and the right pnlnionarv- artery are the only branchou of the trnncus that fall ti> the pulmonary artery, all the other viscer.il-arcii vessels W-'m^ connected wilh the aorta. the left ftflli Tisceral-arcli vessel, therefore, is represontinl in the adult hy the pnlmonaTy artery and the ductus arterioaua. The fchd Kings lieing impervious, only a very sniiill piirt of the lilood of the pahnonury artery is sent to them. Tiie larger ]Kirtioii of the hiiMxl passes fnmi the pnlnionarv artery to the aorla through a communicating trunk, the ductus arterioauB, wliich represents the greater part of the left fifth iireh and whiih lM?eomes imjwrvioiis after birth with the establishmeut of the proper pidmonarv circulation.

These tranafiiimations afford an explanation of the different relations of the recurrent laryngeal nerves of the two sides. At first they are symmetrically arranged. The pnenmogastric nerve, as it crosses the ftinrlh visceral-arch vessel, gives off the recurrent laryngeal nerve, the latter winding around the artery from before backward on its way to the larynx. When the left fourth areh becomes the arch of the aorta and sinks Into the cheat, the nerve is carried with it ; hence after this time, the left nerve is found winding around the arch of the aorta.

Anomsloua airanEements of the branches of the aortic arch, as well .ns of the areh itself, are referable to anomalous development of the original system of visceral-arch vessels. For example, if the right fourth arch, which usually becomes liie right snholavian artery, be suppressed from its origin to the point where the anery for the right upper extremity is given off, the bliKMl must find its way into the latter vessel through the dorsal stem of the fourth arch, and this doi-sal stem will then become the right snbclavian artery. In such case, the right subclavian of the adult will be found to arise from the left extremity of the artih of the aorta and to pass obliquely upward to the right side of the neck behind the tracliea and the esophagus.

THE FETAL VENOUS SYSTEM.

The venotlS Sjrstem of the embryo presents several successive phases, corresponding in part with the various stages in the evolution of the arterial system. The first trunks to appear are the vitelline veins. These vessels have their origin in the vascular area on the wall of the yolk-sac in the manner already described in connection with the vitelline circulation. The two vitelline or omphalomesenteric veins, which result from the convergence of all the venous trunks of the vascular area, follow the vitelline duct into the body of the embryo through the still widely open umbilical aperture and take their course head ward along the intestinal canal to open into the caudal end of the primitive heart-tube (Fig. 82,1, 1). At a later period they open into the sinus venosus of the heart, and still later, when the sinus venosus becomes a part of the general atrial cavity, into the atrium itself. Near their termination these veins communicate with each other by anastomosing trunks that encircle the future duodenal region of the intestinal tube. As the yolk-sac diminishes in size and importance, the vitelline veins decrease in caliber, and the umbilical veins, conveying blood from the allantois and subsequently from the placenta, functionally replace them. The proximal parts of the vitelline veins have an important connection with the circulation of the liver, as will be seen hereafter.

The umbilical veins, which are developed in the mes(Klermic tissue of the allantois, pass from the placenta along the umbilical cord and, entering the fetal body at the umbilicus, run at first along the lateml, and later along the ventral, wall of the abdomen toward the heart. Meanwhile there liave been established a pair of venous trunks, the primitive jugular or anterior cardinal veins (Fig. 84, A), to return the blood from the head and the upper part of the trunk ; and a second pair, the posterior cardinal veins, which bring the blood from the lower part of the trunk, and especially from the primitive kidneys. The primitive jugular vein — which represents the external jugular^ of the adult — passing downward along the dorsal region of the neck, nipets the cardinal vein of it» own aide and unites with it near the heart, the short thick trunk thus formed i>eing the duct of Cnvier. The right and left ducts of Ciivier converge and open tt^ther into the sinuB veuoans (siiiiis reunieus) of the heart, which nlao now receives the vitelline veins and the nnihilical veins. Upon the development of Ihc upper and the lower limhs, the (posterior) cardinal vein appears as if formed by the confluence of the internal and external iliap voin.s, while the primitive jugular below the entrance of the subelaviiin vein is designated, with the duct of Cuvicr, the superior vena cava, since, owing to the preponderance of the jugular over the cardinal vein, the Cuvierian duct appears to be a direct continuation of the jugular. At this time, then, there are two superior venre cava;, the terminal parts of wliieh, however, are not exactly symmetrieid, since the left passes around the dorsal or posterior wall of the atrium, owing to the rotatioQ of the heart from right to left.

---

^ According to Salza (ol)scr nations on guinea-pig) and MaU (observations on human embryo) the external jugular is a secondary vein and the primitive jugular becomes the adult internal jugular vein.

The lower venous tnmks likewise present a symmetrical arrangement. The bilateral symmetry of this stage of the vonuns system, while permanent in fisliec, becomes modified in man to produce the familiar asymmetrical condition of the adult venous trunks by two factors principally — first, the development of an unpaired vessel which is to constitute a part of the inferior vena cava, and second, the atrophy of certain vessels and parts of vessels with a consequent diversion of the major part of their blood-stream into other channels. Associated with these alterations is the evolution of a special set of bIo< id -vessels, the portal venous sfBtem, for the supply of the deveh)ping liver. The development of the portal system, however, may he deferred for separate consideration (see [tage 177).

Wiien the sinus venosns becomes a part of the atrium — constituting thatjiart uf the wall of the adult auricle whieb is destitute of musculi pectinati — the two ducts of Cuvier, or the superior cavse, as well as the veins from the atKloniinal viscera, open by separate orifices into the atrial cavity. An nnpaiied vessel now develops below the heart in the tissue be

THE FETAL VENOUS SYSTEM,

171

tween the primitive kidneys (Fig. 84, A, 1). This vessel is described as growing downward from the ductus venosusnear the point where the latter vessel is joined by the right hejmtic veins (p. 180). It is also described (Lewis) as being formed by the enlargement of the right subcardinal vein, the subcardinal veins being themselves produced by longitudinal anastomoses between veins on their wav from the mesentery to open into the respective cardinal veins. Tiie vessel in

Fig. 84.— Schematic representation of the human venous system, with three successive stages of development (after Hertwig): 1, vena cava inferior; '2, cardinal veins; 3, vena azygos major; 4, vena azypos minor; .">, renal veins; 6, external iliac vein; 7, internal iliac vein; H and 9, common iliac veins; 10, early superior vena* cava?; 11, ducts of f'uvier; IJ, primitive jugular vein; 13, internal jugular ; 14, subclavian vein; l."> and IG, right and Irft innominate veins: 17, vena cava superior; 18, coronary vein; ID, duct of Arantius; 20, hepatic veins.

question constitutes the upper or cardiac .segment of the inferior vena cava. The lower extremitv of this trunk anastomoses by two transverse branches with the right and the left cardinal veins (Fig, 84, 7?). The cardinal veins of the two sides are furtlu^r connected bv a transverse trunk at their lower extremities and bv one that passes across the vertebral column just below the heart. In like manner the two su{)erior vena) cavje communicate with each other by a transverse vessel, the tranflverae jugular vein, at the upper jmrt of" the thorax, above the arch of the aorta. With the exception of the unpaired trunk which is destined to constitute the upper port of the inferior veua cava, the arrangement of the veins at this time is absolutely symmetrical. The apparently meaningless asymmetry of the adult venous trunks is easily accounted for if one notes the alterations in the course of the blood-current wliich now occur.

The blocKl-strcam of the left superior vena cava gradually becomes entirely divertetl into the right cava through the transverse jugular vein, and the part of the left cava below this point, being now functionless, shrivels to an impervious cord (Fig, 84, C). This cord or strand of tissue, the remnant of the left superior cava, is found in postnutjd life, in front of the root of the left lung, embedded in a fohl of the eerous layer of the pericardium, the so-ealletl Testigial fold of Marshall, Since the left superior vena cava receives, near its terminalion in the auricle, the lai^e coronary vein, which returns the greater part of the blond from the heart-wall, this ])roximal extremity of the left cava persists as the coronary sinua of the heart. The transverse communicating trunk — the transverse jugular vein — and the part of the left cava above it now constitute the left imiominate vein, the course of which from left to right is thus exphiincd. The left superior vena cava of the fetus is represented in the adult, therefore, by the sinus coronarius, by the atrophic impervious cord lying in Marshall's vestigial fold, by the vertical part of the left innominate vein and by a part of the left superior intercostal vein.

The lowest connecting branch between the cardinal veins enlarges and conveys to the right enrtiinal vein the blood from the left internal and external iliac veins (Fig, S4j, in consc<|uennp of which the part of the left cardinal vein bctiiw ihu kidney undergoes atrophy and, finally, complete ohliloralitiu. The newly-formed transverse trunk is the left common iliac vein. The part of each cardinal vein above the renal region suffbi's an arrest in growth, in consequence of which the blood is diverted from these veins into the transverse anastomosing branches before mentioned as connecting the respective cardinal veins with the lower end of the unpaired caval trunk (Fig. 84, B and C> 5). As a result^ the lower half of the right cardinal vein, now receiving at its distal end the two common iliac veins, becomes directly continuous with the unpaire<l caval trunk, and with it constitutes the inferior vena cava. The inferior vena cava, therefore, is partly an independently formed structure and is partly the greatly developed lower half of the right cardinal vein. The upper half of the right cardinal vein, conveying now a relatively small part of the blood-stream, becomes the vena azygos major, the termination of which in the superior vena cava is explicable when it is borne in mind that the cardinal and the primitive jugular veins, by their confluence, form the duct of Cuvier.

While no part of the right cjirdinal vein suffers complete effacement, the left one, in a part of its course, entirely disappears. All the blood of the left external and internal iliac veins being transported to the right side of the body through the lowest transverse^ trunk — that is, the newly-formed left common iliac vein — the part of the left cardinal vein Ix'low the kidney retrogrades and disappears. The part of the left cardinal above the renal region lagging behind in growth, the blood from the left kidney is conveyed to the inferior vena cava bv the transverse trunk that (M)nnects the cardinal veins in the renal region ; this transverse trunk becomes, therefore, the left renal vein. Sinee the spermatic veins originally emptied into the cardinal veins, it is found, after these transformations, that the right spermatic opens into the inferior vena cava, while the left spermatic is a tributary of the left renal vein. Some anatcmiists, indeed, regard the left spermatic vein as the representative of the lower part of the left cardinal vein of the fetus.

As the left renal vein develops into the channel for the major part of the blood from the left kidney, the portion of the left cardinal vein above this point remains an inconspicuous vessel, and that part of it intervening between the duct of Ciivicr and the cross branch (Fig. 84, C, 4) situated immediately below the heart undergoes total obliteration. The blood ascending through the persisting part of the left cardinal vein must therefore pass across to the upper part of the right cardinal vein, now the vena azrgos major; and the pervious portion of the left cardinal vein, with the transverse trunk referred to, constitutes the vena asygos minor.

The development of the pericardium is so mtimately related with that of the pleuree ind of the diaphragm that an account of it invohc- a de-icription if the c\olution of tho»e structures. By wa\ of fatilitatuig a cc m prehension of the rather complicated details of the procc ^ the reader is reminded that the tube which constitute-, the primitive heart is formed by the coalescence of the t«o tubes prodiced within the splanchnic mesoderm and that this tube and also, for a time, the heart resulting trom it ire embedded vMthin the ventral mesenterj and further that the part ot the ventral mesentery connecting the heart mth the central

body-wall in the mesocardium anterius \tliile the fdd passing from the heart td the jut tnct is the mesocardium posterius (Fig. 8.'i, A, and F jr 73 () Tht '•| ice betw n the he irt and the Ixxly-wall is a part of the body-cavity or cwloin (throat-cavity of Xcllliker, parietal cavity of His). The first indication of the separation of this space from the future abdominal cavity is furnished by the appearance of a transverse ridge of tissue growing from the ventral and lateral aspects of the body-wall. This mass is the septum transversum. It bears an important relation to the course of the vitelline and the umbilical veins. As the veins diverge from the bodv-wall to reach the heart, thev carrv with them, as it were, the parietal layer of the mesoderm in w-hich they are embedded, forming on each side a fold that projects mesial ly and dorsal ly (Fig. 85, B and C), the two folds approaching and finally meeting witli the ventral mesentery in the median plane. The septum transversum thus formed contains in the region nearer the intestine a mass of embryonal connective tissue which is called the liver-ridge or prehepaticus from the fact that the developing liver grows into it. Since the septum transversum, exclusive of the so-called liver-ridge, is the primitive diaphragm, it will be seen that the liver, in the early stages of its growth, is intimately associated with the anlage ^ of the diaphragm. The septum transversum partially divides the body-cavity into a pericardiothoracic and an abdominal part, as shown in Fig. 85, J5 and C Near the dorsal wall of the trunk, on each side of the intestine and its mesentery, the septum is wanting, and thus the two spaces communicate with each other by openings that are known as the thoracic prolongations of the abdominal cavity. At this stage, then, the four great serous sacs of the body, the two pleural, the pericardial, and the abdominal, are indicated, but are still in free communication with each other.

The pericardial cavity is the first one of these to be closed off; subsequently the pleural sacs are delimited from the abdominal space. Just as the transverse septum, which partly forms the floor of the thoracic cavity, holds an important relation to the course of the vitelline and the umbilical veins on their way to the heart, so is a vertical septum

• Anlage, a German word signifying groundwork, or, in embryology, the

first crude outline of an organ or part, has come into use in English writings upon the subject because there is no exact English equivalent for it entirely distinct from each other. It is evident also, that the mesial wall of each sjjace is constituted by the mesocardium posterius and the dorsal mesentery. The hings first appear as two little sacs, connected by a common pedicle, the future trachea, with the upper end of the esophagus. As they grow downward in front of the esophagus and in contact with it, they push the serous membrane before them carrying it away from the esophagus (Fig. 86, B)y and thus they acquire an investment of serous membrane, which is the visceral layer of the pleura. The layer of serous membrane in contact with the body-wall is the parietal layer of the pleura. The lower extremities of the lungs at length come into relation with the upper surface of the liver, from which organ they are finally se|>arat(Ki by the growth of two folds, the pillars of Uskow, from the dorsolateral n»gion of the body-wall. These folds or ridges projecit forward and unite with the earlier formed septum transvcrsum to complete the diaphragm. So far, however, the diaphragm is merely connective tissue, the muscular condition being acquired later by the ingrowth of muscular substance from the trunk. Occjisionally the dorsal or younger part of the diaphragm fails to unite with the ventral or older fundament on one side of the body, leaving an aperture through which a jx)rtion of the intestine may pass into the thoracic cavity. Such a condition constitutes a congenital diaphragmatic hernia.

The heart and its pericardial sac occupy the greater part of the thoracic cavity, while the lungs arc merely narrow elongated organs lying in the dorsal part of this space as shown in Fig. 86, B. As the lungs increase in diameter, they spread out ventrally and gradually displace the parietal layer of the pericardium (Fig. 86, B) from the lateral wall of the chest, (Towding the pericardium forward and toward the median plane of the body (see Fig. 86, C) until finally the adult relationship of these structures is established.

THE PORTAL CIRCULATION.

The circulation of the adult liver is peculiar in that the organ is supplied not only with arterial bloo<l for its nutrition but receives also venous blood laden witli certain products of digestion obtained frtmi the aliiuentarj' tract, the spleen, and the pancreas. This venoua blood enters the liver thrtj tin- portal vein ami is designed tfi supply to the gland the tnat«;riats for the jwrforinance of its fjiecial functiuas.

duodenum by trunks that encircle the bowel, these connecting vessels collectively constituting the anniilar sinns (Fig. 87, B and C). The liver originates from a small diverticulum which is evaginated from the ventral wall of the intestinal canal. Growing forward between the folds of the ventral mesentery, this little tubular sac divides and subdivides so as to produce a gland of the compound tubular type. The developing liver is fn)m the first in close relation with the vitelline veins and their ring-like anastomosing branches, and receives its blood-supply from the latter through vessels that are known as the ven» hepaticsB advehentes (Fig. 87, 10, 10). These afferent vessels break up within the liver into a system of capillaries, from which the blood passes through the efferent vessels, the venae hepaticse revehentes, into the terminal parts of the vitelline veins. Thus a part of the blood of the vitelline veins is diverted to the liver and, after circulating through that organ, is returned to them further on to be conveyed to the heart. As the liver, with its increasing development, requires more and more blood, the entire blood-stream of the vitelline veins passes to it, and the parts of these veins l)ctween the vena? hepaticw advehentes and the vena; hepaticK) revehentes become obliterated (Fig. 87, B and (,■), The vitelline veins, therefore, leave the intestinal canal at the duodenal region and traverse the liver on their way to the heart. In this early dage of the development of the livery then, it receives its nutrition from the yolk-saCy through the vitelline veins.

When the yolk-sack undergoes retrogression, as it does about the fifth we(;k, the liver must draw upon the allantoic and the placental vessels for its nutrition. To do this it must acquire connection with the umbilical veins. The latter vessels j)ass upward from the umbilicus along the ventral wall of the body and empty into the sinus venosus of the heart above the site of the liver (Fig. 87, A, 4, 4). The umbilical veins effect communications beneath the liver with the vente hepaticaj advehentes from the vitelline veins. At about this time the left umbilical vein begins to predominate over the right one, the latter retrograding until, in the umbilical

the cava, whereby its tributaries, the vena) hepatiese revehentes, come to empty into the cava, the downwanl growth of the latter carrying downward likewise the terminations of these veins to their normal position as the hepatic veins emerging from the dorsal surface of the liver. Meanwhile the volume of blood flowing through the umbilical vein has increased to such an extent that the liver is no longer able to transmit it to the inferior vena cava, and consequently a part of this blood passes through the ductus venosus, which extends from the portal fissure, along the dorsal surface of the liver. The blood of the umbilical vein is divided, therefore, into two streams — one that enters the inferior vena cava directly through the ductus venosus and one that traverses the liver on its way to the cava.

The portal vein results from the persistence of a part of the vitelline veins. The vitelline veins, as we have seen, anastomose with each other by two ring-like branches that encircle the duodenum. The right half of the lower ring and the left half of the up|>er one atrophy, so that the blood of the vitelline veins makes its way to the liver through the left half of the lower ring and the right half of the upper one (Fig. 87, D). The left half of the lower ring and the now united portions of the right and left vitelline veins immediately below constitute the superior mesenteric vein, which passes in front of the third part of the duodenum, as in the adult^ and which is later joined by the splenic vein ; while the anastomosing portion of the loop and the right half of the upper loop become the portal vein. So long as the yolksac is present, the vein receives blood both from it and from the walls of the intestine. After the disappearance of the yolk-sac, the intestinal and the visceral veins are the sole tributaries of the portal vein.

THE FINAL STAGE OF THE FETAL VASCULAR SYSTEM.

The circulation of the fetus at birth and the changes ensuing immediately thereafter may now be easily understood. The fetal blood being sent to the placenta through the hypogastric or umbilical arteries, receives oxygen there and is returned to the body of the fetus through the umbilical vein. The latter vessel takes its course upward along the ventral wall of the abdomen to the under surface of the liver, lying here in the anterior part of the longitudinal fissure. In this position the blood-stream of the umbilical vein is divided into two parts, one of which unites w^th the fetal portal vein to enter the liver, while the other passes through the ductus venosus directly to the inferior vena cava. The blood which enters the liver, after traversing that organ, reaches the inferior vena cava through the hepatic veins. Thus, in the one case directly, in the other case by passing through the liver, all the placental blood reaches the inferior vena cava and passes on to the right auricle of the heart.

From the right auricle the blood passes through the foramen ovale to the left auricle, and thence, through the mitral orifice, to the left ventricle. Being driven from the left ventricle into the aorta, it is conveyed through the branches of the aortic arch to the head and the upper extremities. Finding its way into the veins of these parts, it is returned, through the superior vena cava, to the right auricle, from which cavity it passes, through the tricuspid orifice, into the right ventricle. From the right ventricle it goes into the pulmonary artery. Since the lungs are not as yet pervious, or but very slightly so, the current is deflected almost entirely through the ductus arteriosus to the descending aorta instead of going to the lungs. Some of the blood of the descending aorta is distributed to the various parts of the body below the position of the heart, while some of it is sent through the hypogastric or umbilical arteries to the placenta for aeration! It is evident that no part of the fetal blood, except that in the umbilical vein, is entirely pure, the venous and the arterial blood being always more or less mixed.

With the detachment of the placenta at birth, several marked alterations occur. The circulation through the umbilical vein ceases, that part of this vessel which intervenes between the umbilicus and the portal fissure of the liver becoming, in consequence, an impervious fibrous cord, the round ligament of the liver. The ductus venosus likewise suffers obliteration, becoming the ligamentum venosum Arantii. Since the lungs now assume their proper function of respiration, the communication between the right and the left sides of the heart and also that between the pulmonary artery and the aorta cease. Hence, the respective avenues for these communications, the foramen ovale and the ductus arteriosus, become obsolete. There being no further need for the hypogastric (umbilical) arteries, the circulation through them ceases, and they become mere cords of fibrous tissue, whose ])resence is evidenced by two ridges in the j>eritoneum on the inner surface of the anterior wall of the abdomen. The proximal parts of these arteries persist, however, as the superior vesical arteries.

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CHAPTER XI. THE DEVELOPMENT OF THE DIGESTIVE SYSTEM.

The adult digestive system consists of the mouth with its accessory organs, the teeth, the tongne, and the salivary glands ; of the pharsmx, the esophagus, the stomach, and the small and the large intestine, including also the important glandular organs, the liver and the pancreas. Notwithstanding the apparent complexity of its structure, the alimentary tract may be regarded as a tube, certain regions of which have become specialized in order to adapt them to the performance of their respective functions, tiie salivary glands, the liver, and the pancreas being highly differentiated evaginations of its walls. While in man and in the higher vertebrates the tube is thrown into coils by reason of its excessive length, in the lower-type animals it is much more simple in its arrangement. For example, in certain fishes and in some amphibians the alimentary tract has the form of a slightly flexuous tube, the deviations from the simple straight canal being few and insignificant, and the stomach being represented by a local dilatation of the tube.

The simple condition obtaining in the representatives of the animal kingdom referred to above suggests the likewise simple fundamental plan of the human embryonic gut-tract. There is, in fact, a period in development when the gut-tract of the human embryo has the form of a simple straight tube. The processes incident to the formation of this tube mark the earliest stages of the development of the alimentary system, the tube itself acquiring definite form simultaneously with the production of the body of the embryo.

The first indication of the alimentary canal appears at a very early period of development, being inaugurated in fact by those important alterations that serve to differentiate the blastodermic vesicle into the body of the embryo and the embryonic appendages. It will be remembered that, after the splitting of the parietal plate of the mesoderm into its two lamellae, and the union of the outer of the layers with the ectoderm and of the inner with the entoderm to form respectively the somatopleure and the splanchnopleure, these two double-layered sheets undergo folding in different directions. Before the folding occurs, the germ is a hollow sphere whose cavity is the archenteron and whose walls are the somatopleure and the si)lanchnopleure.^ While the somatopleure in a zone corresponding with the margin of the embryonic area becomes depressed and is carried under that area to form the lateral and ventral body- wall of the embryo (Plate II., Figs. 2, 3, and 4), and also more distally folds up over the arcji to produce the amnion and the false amnion, the splanchnopleure, likewise in a line corresponding with the ivriphery of the embryonic area, is depressed and carried in^-^r^i fr^>m all sides toward the position of the future wimWlk**!?^ This folding in of the splanchnopleure effects t>K^ <^iviMi>n of the archenteron into two parts, a smaller v^x-itv t^Uiuj; within the body of the embryo, which latter iv "SNJnvi^nf «it x\\o same time, and a larger extra-embryonic ^\s*^Yv^itiiw'^t. whioh is the yolk-sac or umbilical vesicle. The Jk-mj -^H*^*;>Nn \>s^^u> \>rtvity is the gut-tract. The constricted ,ss4*,**w*s>ji^v»\^^ Uiw^MMi the two is the vitelline duct. While »K s;',v''^^ v^^^ is still a nither wide aperture, the anterior k^isi 'Ns<*v.-«NSi jv^rts of its intestinal orifice are designated vv^NNv*\v^\ vKs^ i^Wrtor and the posterior intestinal portals.

Vv'*K^ ^NWiMvv^vKsm* oK>st*s in around the vitelline duct, it v^^a. iW NV.4H wt' iho abdomen, the opening left, which is • *.o V <nnI '^^ » ^K^ vi\K U Iviutf the umbilical aperture.

U X . \i\Wii\ \\w\x(\>xy> timt the lining of the gut-tract is

v.. i.s.AxI N> ihs* »uhonmv»t giTm-layer, the entoderm, and

• Iw * i ^\ - V jmlu l»ul clcnunits art* consequently of entodermic

M^ »4 Mk loKliu^ ill of the splanchnopleure begins at

I'v'n iii^ . uvl vU ihv^ <%HHMul week, and is so far advanced

•M'^'i.s 4>. A.»«^. iho -«k44»Hl\«|4%nm« nml the splanchnopleure are not (\> .: . .1 \>- , vU« ix*UU«(^ NS\ut«k bul lU«» |mK*v88es go on at the same time.

no. W.—RHUtisl ructions nf humaq emhrro of ibuut MvuuWen dayi (Ula) ; dd, optic and u(, otic veakloi: ne, nr', nolochord : Mg. hcod-gut; g. mid-gut ; A0. hludKUt; It, Titclllne aBi:; 1. liver: V, lu, ijrlmUlve vcnlrlde and truncu* >HeHcwu>; ni, da. TCDtral and diiraal aortie; on. Hnrlic aruhes; jv, prluitlva jugular vein; cv, cardinal vein; dc. duct of CuvIef; iii'. h>i, umbilical vein and artery: ol, allanUli:

before the end of the third week that the arehenteron nitcly divided int^i the gut-tract and the yolk blastodermic vesicle into the body of the embryo and the embrj'onic apiwndages. It ^v■i^ be remembere<l that, after the splitling of the parietal plate of the mes<xiemi into its two lamellje, and the union of the outer of the layers with the ectoderm and of the inner with the entoderm to form respectively the somatoplenre and the spUnchnopleure, these two double-layered sheets undergo folding in different directions. Before the folding occurs, the germ is a hollow sphere whose cavity is the archeutcrun and whose walls are the somatopleure and the splanchnopleure.' AVhilc the somatoplenre in a zone corresponding with the margin of the embryonic area becomes depressed and is carrie<l under that area to form the lateral and ventral body-wall of the embryo (Plate II., Figs. 2, 3, and 4), and also more distally folds up over the area to produi-e the amnion and the false amnion, the aplanchnoplenrc, likewise in a line corresponding with the periphery of the embryonic area, is (iepressed and I'arried inward from all sides toward the position of the futnre umbilicus. This folding in of the splauchnopleure effects the division of the archenteron into two jjarte, a smaller cavity falling within the body of the embryo, which latter is forming at the same time, and a larger extra-cnibryonic compartment, which is the yolk-sac or nmliilical vesicle. The intra-embryonic cavity is the gut- tract. The constricted commnnioatiou between the two is the vitelline duct. While the vitelline duct is still a rather wide aperture, the anterior and posterior parts of its intestinal orifice are designated respectively the anterior and the posterior intestinal portals.

As the somatopleure closes in artjund the vitelline duet, it forms the wall nf the abdomen, the opening left, which is traversed by the dnct, Iwirig the mnhllical aperture.

It is evident therefore that the lining of the gut-tract is constituted by the innermost genn-layer, the entoderm, and that all its epithelial clementA are consequently of entodermic origin. The folding in of the splanehnopleurc begins at about the end of the second week, and is so far advanced

' Strictly Hi«nkin(t. llie somntopleurc and the splanchnopieiire ire not formed bf/ore Ibe foldmg ocrurti, but the proceues ){o on M Ihe »aaic time.

In its earliest definite form, then, the ^t-tract is a tube extundiug from one end of the embryonic body to the otiier, whii;h o[)ens widely at the middle uf its ventral aspect into the vitelline duct, but which is closed at both ends. It is nsuul U} speak of the primitive gnt-tract as consisting morphihlogically of three jmrts, the head-gut, which is the region on the headward side of the orilice of the vitelline duct; the hind-pit, wliich is the part near the tail-end of the embryo; and the mid-gut or interveniug thinl portion (Fig.' 90).

The closed head-end of the gtit-tuhe corresponds with the floor of the primitive moutb-cavity, the two spaces being separated by a thin veil of tissue, which consists of the enfotlern) and the ectoderm and is called the pharysKeal ffiembrane (Fig. 91). A considerable pniportioii of the so

Pm. tl —Median ncUon Hiroueh Ihe haad nf an embryo nbbit S mm. loDC (kfUtr Mlbklkovical ; rh, membrane bclween alomiKliEUm and rnrc-eut. pharfngeal membrane (Baaheiihiiiit) ; hp, place bam whicb the b^papbysis in iJcvelotidl : A. heart: M, lumen of fore-gut; th. ehonla; v, ventricle or the eerobruui: r", tbird Tenlriele, tliatoflhebclween-braiii (ihalamencephalou) : r<. n>nrlli vcntritle, Ihat of the hlnd-braln and afler-braln (epencephalon aniJ ineteowphalon, or medulla (iblongala): (*, cential canal or the iplnat cord.

called head-gilt constitutes the primitive pharynx. This region of the tube has a relatively large enliher. and presents on its lateral and ventral walls the serios of recesses or evaginations known as the throat-pockets or pttarTngeal pouches (Fig. 71).

While llic inner, entodermic layer of tlic gut-tiilie l)ecomes the inteatinal mucosa, tli<? outer, mesodermic stratum |inulu( the muscular and the connecttTe-tiBBne p:u-t^ of tlii! Imwelwall, the must superficial layer uf tliL- latt*;r with ila mesothelial or ciidotholial ooIIh furmiiig the visceral l&yer of the peritoneum. Since the lueHodermic layer of the gplnnclmupleurc of each side ia coiitiDuoua with the corresponding mcsodemiic layer of the tiomatoplrure on either side of the embryonic axis, the primitive intestinal canal has a broad area of attachment with the dorsal wall of the body-cavity (Fig. 92). Tiio ventral wall is Iikewirie connected with the

Fig. Hi— TtanBvcras seollon of

ventral body-wall throughout the anterior or upper part of its exteut by the continuity of the splnnchnopleuric mesoderm of each side with the somatopleuriu rucsoderm of the fjame side. As development advances, the body-cavity increases in caliber more rapidly than does the intestinal tube, BO that the interval between the two is augmented, in consequence of which the masses of connective tissue uniting the dorsal and the ventral surfaces of the gut with the

Hpouiling woUs of the bod>-caMh become drawn out so as to oontttitute m each case a median vertical foM consibting of two closely approximated lasers of serous membrane with a little toiinective tissue between them These folds are the dorwl and the vsntral msBenteries (Fig 93) While the

Frii. Ml.— Uluinunmallr crofis-sectlons of the body of the embrro In the region of lliu linart nl IbvcI of fliture diaphrtgni : a. eaophagenl Be^nnent of KUt-lnct ; b, (liirHl inuHntury: r, mt'Bocardliiin poaterlua ; d. meioordluiu BnleriuB: c. be^nnliiii iif H-iiliiiii truiiavvrtnni. conlalnlnu vlt«11tne and allamotc veins ; /, aeptum traiiavnnuiii ; a- tlioraclv proliiagation n( abdominal cavity ; nc, neural canal,

(loroul ni(!ri<-nt<^ry extends throughout the entire length of the (Hiiml, till- vt'tirral fold in present only at its anterior or upper IHirt, iiiirn-HjHinding in the extent of its attachment to the dijii'^livc tiilie to that jwrtion representing the future stomach and iip]KT jHirl of the duodenum (Fig. 94). The ventral niirwnU-rv iil tirNi in ]ir('sent throughout the entire extent of the caiinl, Init very early undergoes obliteration except in the Hituiiliiiii iibovi' imU'd. ( '(infeniiiig the reason and the method of ilH diHiippi'iii-iini'i' uotliing is definitely known.

'I'hi- intcMliiiid lulu-, iit a romiKiratively early stage, pre(K'ntH on iN ventral Mirliice near the posterior or cuudat end a smatl evaginiitioii that eidni^s to form the aU&ntolB (see p. 89). While a jKirt of the intra-cnibryonic portion of the allaiitriis diliilon luid develo[>K into the bladder, the part between this latter anil the intestine is known as the urogenital alnuB. Tli(' jtiirt <)f the jrut-tiibe posterior to, caiidad of, the origin of the allantois, is a blind ]ioneli known as the cloa,ca. The latter is, tlicrefori-, the (Mnnnion termination of the urinan,and the intestinal tnicis.

To repeat, we have now, in the thini week of development, the alimentary canal repn'scntwi by a single straight tube

(compure Fig. 90 J, idoried at each eiiJ, bin willi moiitli-cavity an<l anus bolh Judicated, the tube lying within a larger tnhe, the body-cavity, with the walla of which latter it is connected by the dorsal and ventral mesenteries. Along the dorsal wall

FlO. !M,~-R«oi>iutrucUuii iif baiiinn embryo of ■ bout leventeon days (sRer Hli]: 00. optic and nf.otic valcles; ne, uotaehori: hdg, head-gut: p, mid-gut: Aj7. hiadgot: vt, vKollIiio hut; <, liver; e, prInilllTe Tenlrtcle: va, da. renlral and donal «orl«! Jir, primitive Jugblar vein; n. canlinal vein; 'IC. duot of Cuvlcr; iin, ho. umbilical vein aud artery: a', allanloii: Hi. umbillca) cord: dn. dureal mesGUlery; tm, ventral miiaentvry (modified from HIb).

of the body-cavity, dorsad to the parietal peritoneum, pass the two primitive aortic, and later, the single aorta which results from the fueinn of these two. Between the two folds of the dorsal mesentery pass the blood- vessels that nourish the walls of the gut. Within the ventral mesentery are the vitelline veins, which bring the blood from the yolk-sac and VAnwi^y it to the primitive heart. On the ventral wall of the gilt in the wide aperture of the vitelline duct. Farther (JiUHlud, uIho on the ventral surface of the bowel, is the orifice of the iillantoiH. These conditions may be better understood by reference to Figs. 90 and 94. Before tracing the further diivrlopiiicnt of the abdominal part of the alimentary system. It will be pro[)er to note certain very important processes |Htrtiiining to its anterior or head-extremity, and also to conNldor the formation of the anus.

THE MOUTH.

The development of the mouth, the tongue, the teeth, and ilio lUiUvAnr glands has been fully described on pages 134llil. Ill thin connection, therefore, it will be necessary to mil iit((*iiii()ii to only a few of the salient features of their itvoliifloii,

Tim oral oaylty is produced by a folding in of the surfaceiMftodnnn, the fortsa thus formed becoming deeper until it liMMifN llin hcad-itiid of the gut-tract. From the walls of this limMii (Im« NAlivAry glands are developed as evaginations, in the liMiiiiMM' iilnuidy <h»Hcribed, while the teeth are specialized gi'owfliM of itH iM'toderinal lining and of the underlying mesoiJMriii (vlih' p. I.*J7). The first intimation of this infolding U ii|ipiir(Mif at (ho twelfth <lay in the form of a localized ihlitkiMiliiK <» the HiirfjMuwells on the ventral surface of the ImmIv oI* \\\\\ oiiibrvo nrar the head-end. The thickened area in the orftl plato, whieli Hp<»edily becomes depressed, producing tint oral pit or fossa. By the third week, the oral fossa or Nteiuodaum Ih a well-marked pit of pentagonal outline, its buiiiidiirMiM brin^ the niiHolVontal process above, the maxillary proneHHort hitenilly, and the mandibular arches below. The origiiiul oral plate, having n»<M»d(Hl farther and farther from the Hiiriiieo and ioriiiiiig the posterior limit of the mouthmvity, iinw H««piiniteH that cavity from the pharyngeal region of the gut-tube aiui eoines into contact with the anterior wall ot* tlhi hitter. It \h (Milh*d the phanrngeal membrane (Fig. 91). ItH dimippeaniiKu* tM'ciiiN at Home time during the fourth week.

by which event the gut-tube is brought into communication with the mouth.

The exact position of the pharyngeal membrane is not easily definable. It is certain, however, that it falls farther back than the posterior limit of the adult oral cavity, since the primitive mouth includes the anterior part of the adult pharynx. For example, the diverticulum that gives rise to the anterior lobe of the pituitary body belongs to the primitive mouth, yet its vestige, the pharyngeal bursa ^ or Rathkd's [K)cket, is found in the pharynx of the adult. The primitive oral cavity, by the growth of the palate, becomes divided into the adult mouth and the nasal cavities. The hard palate is completed in the ninth week and the soft palate in the eleventh week.

THE PHARYNX.

The pharynx is represented in the embryo by the expanded cephalic end of the primitive gut-tract. It is of greater relative length in the earlier stages of development than later, including as it does, almost half the length of the gut-tube in the fourth and fifth weeks. The primitive pharyngeal cavity is widest at its anterior or cephalic extremity and narrowest at the opposite end, tapering here into the esophagus. Until the breaking down of the pharj^igeal membrane, which takes place in the fourth week, this structure marks the anterior limit of the pharynx and separates it from the oral cavity.

The pharsrngeal poncheB or tliroat-pockets have been referred to in connection with the visceral arches on page 111. They are out-pocketings or evaginations of the entodermal lining of the pharynx, there being four furrows on each lateral wall, and they pass from the ventral toward the dorsal wall of the cavity, each pouch lying between two adjacent visceral arches. The entoderm of the pouches comes into close relation with the ectoderm of the outer visceral furrows (Fig. 71). The mesodermic stratum being

^ It has been shown recently. (Killian) that the pharyngeal bursa is not identical with Rathk^'s pocket, but is an independently formed evagination.

Fig. Wi. Section Ihrougb *Dlage of tonsil of a human fetus (Tourneui) : 1, tonsillar pit, continuous with mouthcsvlly: 2. BeoondBry diverticula: 3. Bolld epltbelial buds ; 4, striped muscular flber.

an evagination of the lateral wall of the pharynx. In the third month the lateral pharyngeal wall jwuches out to form a little foasa (Fig. 95, 1) which is i^itiinted Iwtween the second and thir<l visceral arches, the fossa Ixiing lined witli .tratificd pqiiamous epithelium continnons with that of llie i>lmryngeal cavity. Little solid epithelial buds (Fig. 95) proceed from this diverticulum into the surrounding connective tissue, the buds subsequently becoming hollowed out. Wandering lenkocytes from the neighboring blood-vessels — or, according to some authorities, fnim the mesenchyme cells or from epithelial sources — infiltrate the connective tissue around the young crypts, and these cells becoming aggregated into condensed and isolated groups give rise to the iTmphoidfoUicleB peculiar to the tonsil. The separate and well-differentiated condition of the follicles is not attained until some months after birth. The place of origin of the tonsil lietween the second and third visceral arches explains the position of the adult organ between the anterior and posterior palatine arches, since the latt«r structures represent the deep extremities of the former,

THE ANUS. The early stages of the development of the anus are similar to those of the mouth. The so-called anal membrane is produced by the growing together of the ectoderm and the entoderm, the mesoderm being crowded aside. The site of the aual membrane, or anal plate, is in the median line of the dorsal surface of the embryonic body, at its posterior or caudal extremity. It makes its appearance in the third week. Since the tissue immediately in front — that is, head- ■ ward, of the anal plate projects and develops into the primi

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||ii. i-oirir I I'Tiiiiiiittiori, X\>i-TfU<ri; of the p>tiit<Mirinur\ nyiilriii iind of lln- iiili-xtiiiid 'iiriul.

'I'll.' ■iirliiir ilcjir<*«iofi n-fi-rri-il In atiuvft an llic; anal pit in often called the doacal depression during the time that the cloaca is present. In the lowest mammals, the monotremes, as also in the Amphibia, in reptiles, and in birds, the cloaca is a permanent structure. By the breaking down of the membrane between it and the cloacal depression, it acquires an outlet, through which the feces, the urine, and the genital products find egress. In all higher mammals, however, including man, the cloaca suffers division into an anterior or ventral passage-way, the urogenital sinus, and a posterior canal, the rectum and canal of the anus. This division is eff*ected by the growth of three ridges or folds, of which one grows from the point of union of the urogenital sinus and the gut, while the other two proceed, one from each lateral wall of the cloaca. The three folds coalesce to form a perfect septum. The division is complete at about the end of the second month (or, according to Minot, at the fourteenth week). The cloacal depression or anal pit shares in this division, so that at about the tenth week, it is separated into the anal pit proper, or the proctodeum, and the orifice of the urogenital sinus. The newly-formed septum continues to thicken, especially near the surface of the body, until it constitutes the pyramidal mass of tissue known as the perineal body, or perineum.

The anal pit deepens, the anal membrane being thereby approximated to the end of the bowel, and in the fourth month the anal membrane breaks down and disappears. Persistence of the anal membrane after birth constitutes the anomaly known as imperforate anus.

THE DIFFERENTIATION OP THE ALIMENTARY CANAL INTO SEPARATE REGIONS.

The fourth week marks the beginning of certain important changes in the simple straight alimentary tube. The reader is s^in reminded that this tube is connected with the dorsal body-w^all by the dorsal mesentery and with the ventral wall, for a part of its extent, by the ventral mesentery ; that the canal is, as yet, without communication with the exterior ; and also that the vitelline duct and the allantois are connected with its ventnil surface (Fig. 94). The umbilical vesicle having reached the limit of its development in the fourth week and having begun to shrink, the vitelline duct likewise begins to retrograde and very soon becomes an inconspicuous structure.

The dorsal wall of the tube at a point nearer the head-end begins to bulge toward the dorsal body-wall, forming a somewhat spindle-shaped enlargement (Figs. 97, 98). This di

Afidd/e lobe

of thyroid gland.

Thymus gland.

Lateral lobe

of thyroid gland.

Trachea. Lung.

Right lobe of liver.

Vitelline duct.

Pharyngeal pouches.

Stomach.

Pancreas.

Left lobe of liver.

Small intestine.

• Large intestine.

Fio. 97.— Scheme of the alimentary canal and its acceiisory orj^nB (Bonnet).

latation is the beginning of the future stomach. The part of the mnal on the cephalic side of the stomach lags behind somewhat in growth, corresponding in this respect with the relatively smaller size of the adult esophagus. The esophagus begins to lengthen in the fourth week. At this time, also, the beginning of the liver is indicateil by a small diverticulum which pouches out from the ventral wall of the intestine just posterior to (below) the stomach — the future duodenal region therefore — and which grows into the ventral mesentery. Very soon after the appearance of the hepatic evagination, a similar out-pouching from the dorsal wall of

Fig. 98.— Outline of alimentary canal of human embryo of twenty-eight days (His) : p5, pituitary fossa ; tg^ tongue ; Ix^ primitive larynx ; o, esophagus ; tr, trachea; Ig, lung ; «, stomach ; p, pancreas ; Ad, hepatic duct ; rd, vitelline duct ; a/, allantois ; hg, hind-gut ; W'd, Wolffian duct ; t, kidney.

the future duodenal region of the intestine indicates the beginning of the development of the pancreas.

In the latter part of the third week or in the beginning of the fourth, the esophagus presents a longitudinal groove on the inner face of its ventral wall. This groove increases in depth and caliber and finally becomes constricted off from the esophagus, with which it retains connection only at its pharyngeal end. The tube or tubular sac thus formed is the first step in the development of the lungs and the trachea.

It may be said then that the gut-tract has now, in the fourth week, reached the stage of differentiation into the pharTux, the esophagus, the Btomach, and the intesttne, with the liver, the pancreaB, the respliatoiy sTstem, and the allsntoia fairly begun.

As lieretofore pointed out (p. 90), the allantois — which grows directly from the primitive gut-tract, and which con

alimentury caiial nf huTTinn pmbryo or thIrty-flTe dayi (HIh) : Jib, picuibiry fiiwu: tg. tOQKUv: U. priDiitivv larynx: ", eeo|ib(>KUB : Ir, (rachea; Iq, liing; «, Jlomai-h; ji, pancri.-B*; ftif. hcpalk duel ; e. conira; c(, cloaca; It, kldnuy ; a, anus ; -Tp, genlUt ominonco ; I. cauilal piocesB.

sists therefore of the entoderm and the visceral meaodenn — althoiigli destinetl to produce in part the permanent bladder, functionates for a time, after its union with the false amnion to form the chorion, an an organ of respiration ; while the permanent respiraU)ry pystem, as we have seen, likewise developH from the entodermal epithelium of the guttract The entoderm, therefore, sustains an important relation to the nutrition of both the embryonic and the adult oi^nism. Increase in I^ength and Further Subdivision. —

The intestinal canal grows in lengtli much more rapidly than <loes the embryonic body. It is in consequence of this disproportionate growth that the tube becomes bent and thrown into ooiU or convolutions. During the fifth and sixth weeks a conspicuous flexure appears at some distance below the stomach. Here the bowel assumes the form of a U-shaped tube, the claseil end of the U projecting toward the ventral body-wall (Fig. 100), In other words, the redundant portion

Fig. lOCb— iDtmtniil omi] at Iiuman embryo

of the gut is pulle<] away, as it were, from the dorsal wall of the body-cavity and, as a consequence, the dorsal mesentery is Icngthenetl in this region to a corresponding extent (Fig. 100). The vitelline duct is attached to the piirt of the bend nearest the ventral wall (Fig. 98). At a point on the lower limb of the U the bowel abruptly acquires increased caliber. This dilated part is the beginning of the cacnm or head of the colon, and its appearance initiates the distinction between the large and the small intestine, since the part' of the bowel on the distal side of the point in question becomes also of lai^r caliber and forms the colon.

During the succeeding week or fortnight, the character of the colon and of the csecum becomes better established. The remaining part of the lower limb of the U-loop, with all of the tube included between the loop and the stomacb, is the small intestine, wliicli presents a (iltght dorual flexure at it£ proximal extremity. The stomach meanwhile ha» increatied in size and has almost attaineil its characteristic shape. By the end of the sixth week, then, the alimentary canal has not only increased in length but lias so far dilfereulialed as to have aci)uireil stomacli, dnodetiiun, Bmall intestiiie, cscmn, and Tectnin,

Alteration in the Relative Position of Parts, and Further Development. — The nmst iniportsuit ntudiliciition of llie uliinfutary tube a^ it exists at the end uf the sixth week is effected by certain changes of position of some of its parts. The stomach and the large intestine are the jjortions of the tract most conspicuously affected. The lower limb of the U-segment of bowel, which consists chiefly of the radimentary ctecum and a [Mirt of the colon, is lifted, as it were, over the npfwr limb and comes to occupy a position above it (Fig, 101, ,1], the wecum assuming a posilion in the right

CFm. 101,— Three sucreulre staires ■hnwlng tln> fli'velopnieiil ol the dlgcatlTe tnboand Ihe meaeutcrietln the human IbIiiR{modincdIhnnTounivUii: I.buiduihi l,i1uiH]enum; S, anikll 1ntral1ne:4, colon; 5, vlleUine dnol: B,CKOiim; 7, great Dmentum : 8. mnioiluodcnuni : 9, mrw nt^r? : >", meaocolun. The nrraw polols to the oiiflcc of \h>r nmcntHl buna. The veDlral mespntory In not shown, hyjK vers L

hypochondriac region, and the colon paasinf; thence transversely acros-s the abdomen ventrad to the diiwlenum. This shifting of position on the part of the colon brings about important complications in the arrangement of the mesentery, since the part of the dorsal mesentery that pertains to the upper part of the colon correspondingly alters its position and line of attachment, becoming adherent to the i)eritoneum on the ventral surface of the duodenum. The part of the mesentery in question becomes the transverse mesocolon (Fig. 101, B), The large intestine, after this change of position, presents csecum, transverse colon, descending colon, and rectum, the ascending colon being still absent.

The vermiform appendix in the third month has already acquired the form of a slender curved tube projecting from the cflecum. At the time of its first appearance and for some weeks afterward, the appendix has the same caliber as the caecum. Subsequently the c«ecum outstrips the appendix in growth, the latter appearing in the adult state as a relatively very small tube attached to the much larger csecum.

The caecum soon begins again to change its position, gradually moving downward toward the right iliac fossa (Fig. 101). The downward migration of the caecum is (hie to the growth of the colon in the same direction. In this manner the ascending colon is gradually produced, it having developed to such an extent in the seventh month that the caecum lies below the right kidney, while in the eighth month it passes the crest of the ilium.^ Corresponding with the growth of the ascending colon, the mesentery shifts its parietal attachment and increases in extent until the ascending mesocolon is produced ; and with the descent of the caecum, the terminal part of the small intestine necessarily alters its position to a like degree.

The stomach, up to the third month, is a localized dilatation of the intestinal tube, bulging most in the dorsal direction and having its long axis parallel with that of the body (Fig. 100). In the third month, however, it undergoes an important alteration in position, rotating about two axes. First, it turns about a longitudinal axis, whereby the left side comes to face toward the ventral surface of the body (anteriorly) and the right surface looks toward the spinal column. In addition to the longitudinal rotation, the stom

• According to Treves, the caecum lies under the liver until the fourth

month after birth.

aoh also rotatt^ ui>oii a dorsoventral (anteroposterior) axis, bv which the lower or pyloric extremity moves somewhat iipwTinl ami to the rijrht, and the cardiac end goes tailward ^dowHx^nnh and to the left (Fig. 101). By this double rotatuMi tho stoniavh is made to assume approximately its adult jHv^iuoiu Tlu^ lonjritudinal rotation of the stomach, in which «bo U^xwr jK^rtion of the esophagus takes |>art, explains the t^lMi «t ^« TaffUB nenres in the adult. The nerves, before U^^* rx^i^li^MK Ho one on each side of the esophagus and stom?^^^, bul ^in*v tlu» loft surfaces of both turn forward and the \k\^\\\ MUlJuHw turn Imokward, the left vagus lies on the t^t^v tivM' ^urfa*^* of the esophagus and of the stomach, while vKo M.^hl uorvo Is in rt^lation with their posterior surfaces.

V\w x^dkUimtL of the mesogastrium are influenced in an im|K«tiiOi( uuuuior by the rotation of the stomach. As long as {\w ^\\^\\us\'\\ itiains its original position and relations, with W^ \S,^\^sWv \'\{V\i\U\\v facing dorsad (or |)osteriorly), the mesoyiua^uiu i« a voriiivil mesial fold of peritoneum (Fig. 100), wlulv* iho wntnd mesoutory similarly connects the future \\ y»\'\ \'\\v\'i\\\\\v or vent ml surface of the stomach with the w hii.il b\Hl\ wall At the very beginning of the process of ua-Uhm, tho nir«ogt»ster luHMmies somewhat redundant and

uU'. » t^'waid llio Irl^ O'^K* ^^^y -^)* -^^ ^'^'^ increases in \ slrm, \U\\v \^ ronnod, between the stomach and the dorsal U»mI\ Nsall, a |u»ni^h or |MH*ke(, the omental bursa, whose o|)enu\i\ ti u»\\anl tlh' v\^\\i ( Kig. 101). In the third and fourth iuuulli"« llir ori^riniil unv*4i>gjister, lengthening more and more, i\\\\\ lu'iiu? JilVo*'hHl by the inertMising torsion of the stomach, Hui|i»l"i lu tho \\m\\\ {\f a sae considerably below the level of Clii. ^liiumoli, in fwwxt of (ventral to) the small intestine and {\w l»iiu-.\ri>ir oolt»n. It ultimately becomes the great omeniuur riio nirio^MMiriunu fn)m having been a vertical mesial told, i^ miN\ become a tnmsvers** fold, so nnlundant as to be Ibldrd M|iou it.nrir and to constitute a bag.

\\\ \\\\v nuiuht^r (he ventral mesentery (Figs. 94 and 102), whirli roiuimli llh« anterior or ventral surface of the stomach with tlu' Nrnind iMulv-wall, and in which the liver develops, i^ ulUTfil tVoiu a medial ft)ld to a transverse fold by the rotatiou of the. rttiunai'h. As the liver migrates to a position above the stomach, the part of the ventral mesentery which connects the liver with the body-wall becomes its falciform ligament and coronary ligament, while that portion of this mesentery that connects the originally ventral surface of the stomach, now its lesser curvature, with the liver is the lesser or gastrohepatic omentum. The lesser omentum, therefore, is the anterior or ventral boundary of the orifice of the omental bursa referred to above.

The small intestine begins to exhibit flexures as early as the fifth week, and by the end of the sixth w^eek the duodenum is well indicated as a segment of the gut-tube passing from the pyloric end of the stomach toward the dorsal bodywall. From this time the development of the small intestine, aside from its liistological characters, consists chiefly in increase in length with consequent modification of its mesentery. A striking feature of human development is that, with the growth in length of the small bowel, it is gradually extruded from the abdominal cavity into the tissues of the umbilical cord. The extent to which extrusion takes place increases until the tenth week, after which period the intestine is gradually withdrawn into the abdomen. In the fourth month it lies entirely within the abdominal cavity. Failure of complete restoration of the gut to the cavity of the abdomen constitutes congenital umbilical hernia.

The histolog^ical alterations incident to tiie development of the alimentary tube, from the beginning of the esophagus to the end of the rectum, consist in the differentiation of the constituent elements of its walls from the two strata, the entoderm and the visceral mesoderm, which compose the walls of the early gut-tube. As an initial step in the process, the cells of the mesodermic stratum undergo multiplication and arrange themselves in a narrow loose inner zone and a thicker outer lamella. The inner layer subsequently becomes the submucosa of the fully formed state, while the cells of the outer layer undergo differentiation into unstriped muscular tissue, and constitute the muscular coat of the canal. In the case of the esophagus and stomach, at least, this muscular tunic, in the fourth month, exhibits the distinction between inner circular, and outer longitudinal, layers. The surface-cells oC the mc.':Jodennic stnituiu of tlio primitive stonuich and bowel become the endothelium of the serous coat.

The glands of the entire canal are products of the inner, entoilermic stratuni, and therefore they are intimately related genetically, as well as histologically, with the mucous membrane.

The glands of the Btomach, both the peptic and the pyloric, originate from small cylindrical cell-masses that have been produced by local multiplication and aggregaii<jn of entodeniial cells. By the hollowing out of the cylinders and the branching of the tubes thereby formed, the two varieties of gastric glands are evolvetl. Both sets make their appearance in the tenth week. Until the fourth month the peptic glands contain cells of but one tyi>e ; at this [leriod, however, certain cells of these glands becurae altered bv the gradual accumulation of griinules within their protoplasm, by which they arc trnnsfurmcd into the ehurac (eristic acid or parietal calls of these glands.

The glands and villi of the intestine are likewise products of the entodermal lining f>f the gut. Their evolution begins in the second month, and they are fairly well formed by the tenth week. As in the case of the gastric glands, the glands of the bowel develop from cylindrical masi^es of entodermal cells which are at first solid, hut which later become hollowed out to form tubular depressions or follicles. In the regioii corresponding to the upper part of the small intestine many of these follicles branch to give rise to the gl&nds of Bmnner, while tmbranched, simple, tubular depressions distributed throughout the entire length of the bowel become the glands of LieberklUui. While the surface entoderm is thus growing into the underlying mesodermic tissue to form the glands, it becomes elevate<l into nnnnte projections between the months of the gland-ducts, forming the villi of the intestinal mucosa. The connective-tissue core of the villus is derived from the »ndprlyini» mcsmicrmic tissue, the cells of which, proliferate tng, grow forth into the entoderm. The villi at first are present throughout the large and the small intestine alike, being well developed by the fourth month. While the villi of the small bowel continue their development, those of the large intestine, after the fourth month, begin to retrograde. At the time of birth they are still discernible, but at the end of the first month after birth they are completely obliterated.

Meckel's Diverticulum.— The vitelline duct, it will

be remembered, is the avenue of communication between the early gut-tube and the umbilical vesicle. In the sixth week the umbilical vesicle has already begun to retrograde, and the vitelline duct is attached to the ventral extremity of the U-loop of the bowel present at this stage. The vitelline duct in most cases suffers complete obliteration in the later stages of fetal life. In some instances, however, its proximal extremity persists in the form of a small blind tube varying in length from one to several inches, which is known as Meckel's diverticnlunu Since the site of attachment of the vitelline duct is not far from the termination of the small intestine,- Meckel's diverticulum, when present, is connected with the lower part of the ileum, at a point from one to three feet from its termination. Should this tube remain attached to the umbilical aperture and retain a patulous orifice, there would result a congenital fecal fistula.^

THE DEVELOPMENT OF THE LIVER.

The essential features of the dev^elopment of the liver will be more easily apprehended if the reader will not lose sight of the fact that the organ is a compound tubular gland, and if, further, he will recall the method by which glands in general are developed — that is, as evaginations of the wall of the cavity or organ to which they pertain.

The first step in the evolution of the liver is the growth of a diverticulum from the ventral wall of the gut-tube at a point corresponding to the region of the future duodenum. This occurs in the third week, since His found the diver

• Meckel* 8 diverticulum is of interest clinically, since by contracting

adhesions to adjacent coils of intestine or by entanglement, it may produce acute obstruction of the bowel.

abundant cell-proliferation. The numerous branches into which they divide are not tubes, but solid cylinders of cells, the hepatic cylinders. The secondary branches of these cylinders unite with corresponding branches of adjacent systems, producing thereby a network of inosculating cell-cords, the meshes of which are occupied by young connective-tissue cells and the developing bloodvessels. The connective and vascular tissue of the liverridge, thus surrounding and permeating the epithelial cellcords, produces all the connective-tissne parts of the liver, while the liver parenchyma — the proper hepatic cells — and the epithelium of the bile-dncts originate from the primitive entodermic evagination. The cords of cells are in part hollowed out to form the bile-ducts and bile-capillaries, and in part become the cells of the lobules. The cylinders that are to produce the bile-ducts acquire their lumen by the fourth week.

Until the middle of the fourth month, the right and left lobes of the liver are of equal size, but after this period the right lobe outstrips the left in growth. The liver grows very rapidly and is relatively of much greater size in the fetus than in the adult, almost filling the body-cavity at the third month. In the later months of pregnancy it reaches almost to the umbilicus, while at birth it makes up oneeighteenth of the body- weight.

The £fall-bladder develops as an evagination from the original diverticulum. It is present in the second month. The pedicle of this evagination lengthens somewhat and becomes the cystic duct. The stalk of the hepatic evagination itself becomes the ductus communis choledochus.

The ligaments of the liver, save the round ligament, are simply folds of the peritoneum which connect the organ with the abdominal wall. Falling into the same category, though not usually designated a ligament, is the gastrohepatic omentum, which connects the liver with the stomach. These various peritoneal folds may be looked upon as parts of the ventral mesentery. Since the liver evagination grows between the two layers of the ventral mesentery to reach the septum transversum, the liver will be found, iti the early stages of its development, embedded between the lamellte of tliis mesentery, which in a riiedian vertical fiild of jxritoneum (Fig. 102). The liver is therefore enclosed iu the peritoneum and is connected below, by a part of the ventral mesentery, with the lesser curvature of the stumiich, which still lies in the median plane of the body, and above and in front, with the diaphragm and the ventral body-wall by the upper and anterior part of the same structure. The latter fold is somewliat modified by the intimate asi^ociation of the early stage of the liver with the primitive diaphragm, the liver having develi)i>ed within a ]H>rtiori of the septum transversum, the liver ridge. As development advances, a partial separation of the liver and the diaphragm is effected, the peritoneum, as it were, growing between the two from both the ventral and the dorsal edges of the liver. The region which is not invaded by the peritoneum represents the nonperitoneal surface of the adult liver between the lines of reflection of the two layers of the coronary ligament. Since the peritoneum on the under surface of the diaphragm is reflected from that muscle, both in front of and behind this area of contact, to liecome continuous with the peritoneum on the convex surface of the liver, there are formed two transverse, parallel, but separated, folds which constitute the coronary ligament of adult anatomy. The lateral prolongations of these foldp to the lateral wall of the abdomen constitute the lateral ligaments of the liver.

The nitation of the stomach to assume it^ i)ermanent relations alters the position of the fold that ci>nnect8 its lesser curvature with the liver, bringing this fold into a plane parallel, approximately, with the ventral wall of the abdomen. This fold is now the lesser or gastrohepatlc omentmn.

The ronnd ligament of tlic mlult represrnls the impervious vestige of the umbiliiid vein. This vessel, entering the fetal body at the umbilicus and pa.ssing to the under surface of the liver, diverges from the abdominal wall to reach that organ and. in doing so, carries with it the piirietal iK'ritoneiim, The fold thus formed is the falciform or suspensory ligament.

The special system of blood-vessels belonging to the liver is described in the chapter on the Vascular System, p. 177.

THE DEVELOPMENT OF THE PANCREAS.

Until recently it was believed that the pancreas developed from an ovagination of the dorsjil wall of the gut-tube in the region of the future duodenum, opposite the site of the hepatic diverticulum. Later investigations have shown, however (Stoss, Hamburger, Brachet, and others), that three evaginationSy one dorsal and two ventral, appear upon the wall of the duodenal region of the gut-tube, the method of development being strikingly similar in mammals, birds, fishes and ampliibia.

Fig. 103.— Reconstruction of duodenum with pancreatic diverticula (after Uamburger) : A^ Five weeks' embryo ; It, six weeks' embryo ; D, duodenum ; D.chol., common bile duct; V.P, ventral pancreas; D.P, dorsal pancreas; JT, point of fusion of the two ; .S, stomach.

In the sheep a dorsal evagination appears in a 4-mm. embryo (Stoss), and somewhat later two ventral outpouchings apj)ear in close proximity to, if not in actual connection with, the hepatic diverticulum, the stalk of which latter becomes the common bile-duct. The dorsal diverticulum penetrates between the two layers of the mesogastrium (Fig. 103) and gives off lateral branches, remaining attached to the dorsal wall of the duodenum by its stalk or duct. Eventually tliis system of branching epithelial tubes, the dorsal pancreas, becomes the body and tail of the adult pancreas.

The rigid and left ventral evaginations become confluent and form the vential pancreas. According to some authorities the left diverticulum atrophies, the right alone persisting to form the ventral |)ancreas. In either case the stalk or duct of this ventral fundament bw;oinc^ tnorgfxl into the eommoJCQ bile-duot — if not jireviously eouaected with it — so thatit i^ in effect, a bramli of that dnct. The ventral pancreas grow to the left, in front of the portiil vein, this change beinj favored by the rotation of the duotlcniim on its long i penotmtos between the kvei-a of the nicsogastritini and fuses with the dorsal pancreas (Fig. 103), hecomiiig the head of the adult oi^n. Thin onioo occurs in the sixth week in man (Hamburger). Willi the union of the two |iortions their respeetive ducts— the dorsal duet or duct of Santorini and the ventral or duct of Wiranng — acquire auaslomosefi with each other, after wliieh event the terminal or intestinal part of tl duct of Santorini atrophies and disappcais, the duct of Wi: suug being heneefiirth the avcune by wich the later-estate ]ishe<l .'ieeretinn enters the dmiflenurn. Occasionally entire duct of Santorini jiersisls to adnlt life, entering t duodenum upon its dorsal wall. In the cow and pig I ventral duct atrophies, the duct of Santorini alone per&istin^a while in the horse and dog both duets persist.

What has been said above applies to the origin of thftl epithelial ])ai'ts of the gland ; the coimectlTe-tissne and i lar elements are of mesnderniic origin.

At six weeks the long axis of the pancreas nearly c sponds with that of the body of the fetus. With the rota-^ tion and change of position of the stomach and the alti tionft in the mesogastrium, it moves to the left, acquiring its 1 permanent relations with the lert kidney and the spleen. It ] continues to be an intraperitoneal organ until the fifth month, when, by the disappearance of the tlorsal ]iart nf its investment, it becomes retr(i[>eritoneui (Fig. lOfi, ,1 and H).

THE DEVELOPMENT OF THE SPLEEN.

Although the sjileen dm-s not belong to the digestive system, it may convenienliy lie eonsidcrid here because of its position and relations.

This organ is differentiated from the meaodermic tissue (me.se nchy ma) found between the layers of the mesogastrium in close proximity to the developing pancreas (Fig. 102).

Primitively, therefore, it is situated behind the stomach. The first step in its development, recognizable at about the end of the second month, is the accumiilation of numerous lymphoid cells with large granular nuclei. The origin of these cells has been a matter of dispute. It has been asserted (Maurer, Kupfer) that they come from the epithelium of the gut-tube, but this is denied by most authorities. The findings of Laguesse in fish-embryos, demonstrating the origin of the spleen aniage from mesencliyma in close relation with the branches of the later portal vein, are possibly significant in view of the relationship between the spleen and one of the largest tributaries of the porta\ vein, namely, the splenic vein. Tonkoff's observations on birds and mammals (1900), confirmed by Hochstetter, reaffirm the mesenchymal origin of the spleen.

The mass of cells is augmented by the addition of cells immediately beneath the peritoneal surfaces of the mesogastrium, which cells elongate imtil they are spindle-shaped and then become aggregated into fusiform masses. Bloodvessels penetrate the fundament in the third month and become surrounded by cells of the same spindle-shaped type. From both the cells surrounding the blood-vessels and from those of the fusiform aggregations, processes grow out and unite with each other, and from the network thus formed the trabecular framework of the organ is ultimatelv evolved. Accumulations of small nucleated cells, forming dense masses along the arteries, furnish the chief constituent of the pulp. The delicate intercellular substance which makes up the remainder of the pulp is filled with blood-corpuscles. The Malpighian corpuscles appear before the end of the fourth month. By the sixth month, the spleen attains its characteristic shape and the fibrous capsule is clearly indicated.

The spleen undergoes a change of location coincident with the rotation of the stomach and the alteration of the mesogastrium. The organ being from the first embedded within the mesogastrium, it follows that peritoneal fold to the left side of the abdominal cavity. Here it lies close to the cardiac end of the stomachy between the two layers of the mesogastrium, but projecting toward the left. The part of th(» mcsogastrium which intervenes between the spleen and th(* stomach is the gastrosplenic omentum ; while the part that pass<;H from the spleen to the posterior wall of the abdomen, r(»prcs<»iiting the parietal attachment of the mesogastrium, (^onstitiitcH the phrenicosplenic omentum.

THE EVOLUTION OF THE PERITONEUM.

The arrangement of the peritoneum being subordinate to thc! pcmition and relations of the vis(»era contained within the iilidonion, th(! development of this complex membrane can be pro|M?rly <lcwTib(Hl only by tracing the growth of the digestive HyHlnii. Am the formation of the early gut-tul>e by the infolding of tlu; splanchnopleure has been pointed out (pp. I HO iiiid IH8), w<» may begin at once with the i)eriod when fhi' IriM'l luiH ilir form of a straight tube connected with the doi'.-iil niid iIm' vontPid IxKly-wall respectively by the dorsal iiiid I III' ventral mesentery (Fig. 104). Covering the tube as a roih'shtiiriit purl of its wall, is the splanchnic or visceral layer 111' llir iiiiv<^od('rm, while the somatopleuric or parietal layer ol' ilii« lallrr lificM th(» wall of the bodv. Obviously these Iwo liiiiirllip of llic incsfMlcrm are continuous with each other tliroiipfli \\\i* iiM'diiim of tlu* mesenteries mentioned above (ll^i. lO/i, .1 Mild //). The space thus enclosed by the mesodrniih* nlnitn Im the body-cavity or co»loin or pleuroperitoneal nu\M\. TIm' ^iirfiUM'-cells of both strata flatten and assume llin rliiinirtiT of mcsothclial, the later endothelial, cells. If, at thin ntit^e, one iN'gins at any point to trace the mesothelial lining, of the body-<'avity, that lining is found to form praclirallv oiiti ctMitinuous sheet.

Tliin hiiiiple nrrangenuMit of the primitive peritoneum is (ninntonnrd into the (Mimpli(*ated membrane of the adult, prinmrily, by tin* inerejiHe in length and cons(»quent tortuosity id' tht) ulinii'iitiirv tube; and, sec<mdarilv, bv the fact that iH^i'tain oppoM'd portions of the s<tous membrane, which have been brought int<» eontaet by the altered relations of the bowel and tlu* stomach, undergo con<»reseence or fusion with each (itlior. Siiiiiihancoiialy wilh ihcsc alterations, [he original pIeuroperi1»neal cavity sufl'ci's division into tlic abdominal or peritoneal cavity anil the thoracic part of the body-cavity by the devclupinent of the (lia[»luiigni. This is described on p. 175.

Fin, im —Recoiislruplion of Immun embryo of »boiilaeTtnteen(l»yB(«flerHW: OP, opl[c and of, otic vinlclcs: nr. imtuchord : Mg, head-gut: g. mid-gut: lig, hindgut; T,, vitelline aac; I, liver; v, primitive venlrtcle; m. da. ventral and doraal «i)iniE:Jp. primitive Jugular vein; ™, cardlual vein; rfC, dnct of CuvlBr; m, no, umlilUcal vein and artery; at, allaniaii; ua. umbiilcal eord; dm. doiMl meKDtery; vn, vcntnl meneiitvry Imudilled (mm Hlsi.

Tho first modification of the original arrdngemeDt is effected by tiie development of the stomach a^ a spiiidlfi-shaped <lilatation of lh{' gul-tiil>e, ditferentiatiiig the tube int*) the stomach and the intestine, and the c-omniuD dorsal mesentery

into the meBoeastrinin and lite iatestmal mesentei?. The drawing out of the U-shu])cd liwp of ihc inti'.-^tiiic from.the dorsal body-wall, which is the prolirniiiary siep to the distinctioD between the small iulcstinp and the colon, iiiL-reaspa the length of the iiitcHtinal raesenlerv to a corresjwnding extent (Fig, 10'>, C). As heretofore indicated, the lower limb of the liKip presents an enlargement which is the beginning of the development of the lai^e intestine.

Km. los -A. B. two tniuvene sectlona, A Ihniiii Intl regjao: C aogiltal aecKon (Toumcux) : l, clone lerr; H, mewKardlum poalerlui: 4, inesocanlliiia anterliu: !>, lumer omentum ^ 6, ■UBpensory ligament uF tlvcr; 7. eaophague; 8. lungs: H. bi»rli ID, puncnu; 11, itnmnch: 12, llrer; l:i, apleen: 11, loop of lulcatlno vilh Tltclllue duct: IS.ciccum; IB. trecbcL

An important stage in the evolution of the jieritoneum is tnarkod by the rotation of the stomach and by the migration of the proximal part of the largo intestine to a new location. The eliange of position on the part of the colon may perhaps be best expressed by saying that the U-Ioop of intestine rotates upon an obllqne dorsoventral axis, whereby the lower limb of the loop, in other words, the termination of the small bowel and the l>eginning of the colon, is carried to a positioD above, cephaliid to, the nppor limb (Fig. 101, A), This rotation brings the beginning of the colon into the riglit hyjK)chondriac region of the abdomen, from which point the transverse colon passes across the abdominal cavity, ventrad to the proximal end of the small intestine or duodenum. As a consequence of the altered situation of the transverse part of the colon, its mesentery shifts its area of attachment by fusing with the peritoneum of the dorsal wall along a horizontal line and also with that of the ventral surface of the duodenum. The descending colon having meanwhile moved to the left, its mesentery likewise acquires a new area of attachment by concrescence with the parietal peritoneum of the dorsal wall of the abdomen on the left side. During the progress of these alteraticms, the small intestine increases in length, and its mesentery becomes correspondingly more voluminous both in the extent of its intestinal border and in length. The convolutions of the small intestine now occupy the space below the transverse colon and its mesentery.

The duodenum, which in the early stage shares with the gastro-intestinal tube in the possession of the common dorsal mesentery, loses its mesenterial connection with the abdominal wall and becomes thereby a fixed part of the intestine. Mention was made above of the fusion of the transverse mesocolon with the peritoneum of the ventral surface of the duodenum. At about the same time, the duodenal mesentery (Fig. 101, A) fuses with the parietal peritoneum of the posterior abdominal wall, the result being that the lower layer of the transverse mesocolon, as it passes downward, is now continuous with the parietal peritoneum, there being no longer any serous membrane between the transverse part of the duodenum and the abdominal wall (Fig. 106, J5). This part of the duodenum therefore becomes retroperitoneal, there being an investment of serous membrane only on its anterior or ventral surface.

The second modifying DEietor in the complication of the peritoneum, the rotation of the stomach, initiates alterations in its mesogastrium. The latter membrane, it will be remembered, is a vertical median fold of peritoneum continuous with the mesentery of the duodenum (Fig. 105, C).

As the stomach tcovca about its two axes of rotation, the mesogttstriuni begins to eaj^ toward the left (Fig. 101), sothat now it constitutes a pouch or foasa, the omental bursa, situaated between the stomach and the dorsal body-wall, the opening of wliich looks towartl the right side of the body (Figs. 100, ^1 and 107). Wilh the rapidly increasing rednndaney of the niesogastriTnii, the omental bun-a become.s

Fig. V».—A, B. Tw

more and more capacious. In correspondence with the progressive rotation of the stomach, what was at first the left surface of the mc>sogastriiim comes Into contact with the peritoneum of the dorsal abdominal wall and fuses with it, thus changing its area uf parietal attachment fnim a'median vertical Hue to a transverse one. This change is (Mimpleted by the time the :«tomach has attained its normal adult jmsition. The omental bursa now has the position and relations shown in Fig. 106, A, 8. A still further increase in the size of the bursa results in its protrusiim downwanl in front of, ventrad to, the transverse colon and the small intestine. Kefence to Fig. 106, B will show that the dependent part of the lnirs;i very nearly corresponds with the fully formed great omeDttiro. It will be seen, however, that the deeper layer of the bursa, the layer nearer the intestines, may be traced above the transverse colon and its mesentery to the dorsal wall of the abdomen, where its two lamellie separate to enclose the pancreas, one lamina passing over the ventral surface of the pancreas to become continuous with the parietal peritoneum, while the other layer passes between the pancreas and the abdominal wall. The latter layer is in continuity here with the i»arietal peritoneum, which almost immediately leaves the abdominal wall to form the upper layer of the transverse mesocolon.

The further alterations necessary for the attainment of the completed condition consist in the coD<3«Bcence of certain opposed peritoneal snrfaceB. As a conspicuous example of such concrewcnce, the deeper lamella of the layer of the omental bursa just dcicribed fuses with the ventral jxritoneal surface of the transverse colon and with the upper layer of the transverse mesocolon (Fig. 106, A), after v^hich event this deeper lamella is practically continuous with the lower layer of the mesocolon, while the superficial lamella is in continuity with the upper layer of the mesocolon (Fig. 106, B), Thus the transverse colon appears as if enclosed between the two lamcllic of the deeper layer of the great omentum, while its mesocolon is constituted by a part of the same structure. In other words, the adult transverse mesocolon includes not only the primitive membrane of that name but also a part of the early mesogastrium. Similarly, the opposed surfaces of peritoneum between the pancreas and

iUt*iltirm\ iilHlcmiinalwall undergo fusion (Fig. 106), the effect of wlii<*|i, after the concrescence of the mesocolon with the <l<tif|Mfr lay^'r of the omental bursa, is to make the lower layer of iUt*, in(*M<K*f)lon continuous, over the transverse part of the <iM#HliffMitii, with the parietal peritoneum.

I ii« ffreat omentum of descriptive anatomy, resulting from Mi<< downwardly projecting process of the omental bursa, i'^malMn originally of two layers of membrane, each one liuvin^ two wTous surfaces. At the time of birth these iwtf UtyovH uro, ntill separate — the permanent condition in ^*ini' tMuiMfiialH— l)ut during the first year or two after birth imy \u*iuniu» adherent, the great omentum thus coming to iunii\ir\tui but a ningle layer.

it rttitHiuiti to note the metamorphosis of the ventral mesen tWVi H bii'h, prior to the rotation of the stomach, is a vertical

liMfdiaii fold ronnecting the lesser curvature of that viscus

with i\m vt'Ofnil ulHlominal wall. Since the evagination of

tUii |iiif-(iibi; (hut given rise to the liver grows between the

liiytii'D of \\w. \'i*ulvii\ ni(?sentery to reach the septum trans Vi^reMnii (Ih^ liv<*r in not only enclosed by the mesentery, but

}H roiiiH'rh'd by it with the stomach and with the ventral

wall of the lilNlonii'ii and also with the primitive diaphragm

(Kij/. lorj). l^y tin* rotation of the stomach, the vortical

nM'diaii (old whirh connrrts that orgjin with the liver becomes

ho allrnd in pohitinn an to \\r in a plane approximately par alhJ with lh<^ vi'nlrnl hurfarr of the body. This fold is now

till' gatttrohapatic or lesser omentum. As referenc'c to Fig.

lOtI will hhow, il iri (hf anterior boundary, above the position

of thii ht4»nnu*h, of tint nac deH<Tilwd above as the omental

burrtu.

That pari of thi^ ventral incHentery that connects the liver with the abdominal wall antl with the diaphragm, while originally orfupyiiig the inetlian plane, is modified by the relation of the <h»v<«loping liver to the primitive diaphragm. thene organs are intiniati*ly united with each other (p. 175) in the early stjige <»f tlifir growth, l)Ut with their completion a Hi'pamtion takrs place. V\h)u the two separated surfaces, except in a region near the dorstd wall, the cells assume the endothelial type, the opposed surfaces thus acquiring the characters of serous membrane. The peritoneum on the under surface of the diaphragm is continuous with that on the upper surface of the liver, both in front of and behind the non-peritoneal area of contact. Therefore, in the completed condition of the liver and the diaphragm, these two structures are connected by two layers of peritoneum separated from each other by a region containing only areolar tissue. These layers constitute the coronary ligament of the liver. If now Fig. 106 is inspected, it will be seen that the posterior layer of the lesser omtMitum, and the upper layer of the transverse mesocolon, together with that part of the peritoneum with which they are in direcjt continuity, enclose a sac which is the so-<^alled lesser bag of the peritoneum or the lesser peritoneal cavity. All other parts of the peritoneum taken together constitute the greater peritoneal cavity. The communication between the two, the foramen of Winslow, situated behind the free right border of the lesser omentum^ is the constricted orifice of the early omental bursa.

The position of the kidneys and the ureters as retroperitoneal structures and the relations of the bladder and of the uterus to the peritoneum, encroaching as they do upon the parietal layer of this membrane, and being, therefore, invested by it to a greater or less extent, are easily accounted for when it is recalled that all these organs develop from the somatic or outer layer of the mesoderm.

The peritoneum does not acquire all the characteristic features of a serous membrane until about the third month. The histological alterations begin in the fourth week, from which time until the sixth week the superficial cells, the mesothelium, pass through various phases of transition to reach the condition of somewhat flattened elements. By the eighth week they have acquired the form of true endothelium. It is not, however, until the third month that the subjacent tissue has attained to the condition of a fullyformed basement membrane.

+++++++++++++++++++++++++

CHAPTER XII.

THE DEVELOPMENT OF THE RESPIRATORY

SYSTEM.

Although the Dasal chambers and the pharyDgcal cavity contribute to the formation of the respiratory system, these

Middle lobe

of thyroid gland.

Thymus gland.

Lateral lobe of thyroid gland.

Trachea, Lung.

Right lobe of liver.

Vitelline duct.

Pharyngeal pouches.

Stomach,

Pancreas.

Left lobe of liver.

Small intestine.

Large intestine.

Fig. los.— Scheme of the alimeiiUry canal and its accessory organs (Bonnet).

jiarts will not be considered here, since they are described elsewhere.

Anatomically and according to their mode of development^ the lungs might be looked upon as a pair of glands having a common dnct, the trachea, which latter, through the medium of its dilated proximal extremity, the larynx, opens into the pharyngeal cavity. In point of fact, these organs are developed as an ontgrowth from the entodermal alimentary canal in a manner similar to the development of the liver and the pancreas.

The first step in the development of the lungs is the outpouching of the ventral wall of the esophagus throughout its entire length. The longitudinal median groove thus formed is the pulmonary groove. It makes its appearance when

the embryo has a length of 3.2 mm. (0.128 inch) or probably early in the third week. The groove is more pronounced at its lower or gastric extremity. As the groove deepens, its edges approach and finally meet and fuse ^vith each other. In this manner the groove is converted into a tube, wliich gradually separates from the esophagus, the separation beginning at the end toward the stomach and progressing toward the pharynx. The separation, however, is not complete, stopping short of the upper end of the groove, so that the tube retains communication with the pharyngeal end of the esophagus. Even l>efore the constricting oiT of this tube or pulmonary diverticulum is completed, its free end bifurcates. The pulmonary anlage consists, then, at this stage, of two short wide pouches connected by a common |>edicle with the primitive pharynx (Figs. 108 and 109), and this condition is present in the fourth week.

A'^ery soon after the end of the first month each of the pouches undergoes division, the right one into three branches,

Fkj. 109.— Transverse section to show outgrowth of pulmonary anlage from gut-tube (alter Tourneux): 1, dorsal mesentery; 2, ventral mesentery ineluding 3, mesocardlum posterius ; 4, mesocardlum antcrius; 7, esophagus;

8, diverticulum which becomes the lungs, the trachea, and the larynx;

9, heart.

THE THYROID AND THE THYMUS BODIES,

225

epithelial cylinders, the lumina being acquired later. At first, the lining entodermal cells of the primitive tubes are tall and cylindrical, the tubes themselves having a relatively small lumen. In the fourth month the cells acquire cilia. From the anatomical standpoint, the lungs now present the characters of compound saccular glands.

From the sixth month to the end of gestation occur the changes which give to the organs their essential characteristics. Upon the dilated extremity of each terminal tube numerous little evaginations develop. These are the air-sacs, or pulmonary alveoli, the terminal tubes from which they are evaginated being the alveolar passages and the inftmdibnla. Their walls remain very thin and their lining epithelium flattens to such a degree as to closely resemble endothelium. The trachea is simply the elongated stalk of the pulmonary diverticulum. Its incomplete cartilaginous rings first appear in the eighth or ninth week.

The larsrnx is the dilated proximal extremity of the stalk of the pulmonary diverticulum specially modified to serve as an orgjin of plionation. It is first indicateil at the end of the fifth week (or, according to KalHus, in the fourth week). One of the earliest changes is the appearance of two dorsoventral ridges at the junction of the primitive tnichea with the esophagus. They are close together in front, ventral ly, but separated dorsally. They are the first indications of the true vocal cords. At this time the pharyngeal aperture of the primitive larynx is at about the level of the fourth viscenil furrow, behind the three segments of the developing tongue (p. 144), and is separated from them by the Aircnla, a horseshoe-shaped ridge which bounds the aperture in front and laterally and which represents apparently the ventral parts

15

com

Fio. 112.— Entrance to larynx in a forty- to forty-two-<luy human embryo (from KalliusK /, tuberculum impar; p, pharynjro-epiglottic fold; e, epiglottic fold : l.e, lateral part of epiglottis ; ni. cuneiform tubercle; com, cornicular tubercle.

of the third visceral arches (Fig. 71, A, 3, p. 144). A little later the furcula difTerentiates into a median eleratioii, which is the anlage of the epiglottis, and into the two lateral arytenoid ridges, each of which latter presents two little elevations^ the comicnlar and cnneiform tabercles respectively (Fig. 112). The arytenoid cartilages are thus well indicated by the sixth week. The lateral p)rtions of the furcula also produce the aryteno-epiglottidean folds.

The thsrroid cartilage develops in two lateral halves from corresponding masses of mesenchyme which chondrify from two distinct centers for each mass. It is regarde<l as representing the cartilages of the fourth and fifth branchial arches. The two alie fuse with each other ventrally as development advances. Failure of cartilaginous iniion between the two alo) constitutes the malformation, /oramr/i thyroidcum. The cricoid cartilage is regarded as being an independent cartilaginous formation in scries with the rings of the trachea. Tiie chondrification of these various elements of the larvnx begins in the eighth or ninth week.

The development of the pleurae has been descril)ed in connection with that of the ]x»ricardium and of the diaphragm (p. 175).

THE THYROID, THE PARATHYROID, AND THE THYMUS BODIES.

Th(\*<e organs may be considered in this connection as a matter of convenience and because of their embryological relationship to the* respiratory system, being developed, like the latter, from the epithelimn of the gut-tract.

Th(» thyroid body, an organ common to all vertebrates, genetieallv consists of two parts, a median and two lateral portions, or lateral thyroids.

The median portion originates from an evagi nation of the ventral wall of the pharynx, in the median line, posterior, caudad^ to the tuberculum imi)ar, and between the ventral extremities of the first and second visceral arches. This median diverticulum is present in the human embryo of 5 nmi. It soon pouches out on either side, assuming thereby the form of an epithelial vesicle connected by the constricted pedicle of the diverticulum with the ventral wall of the pharynx (Fig. 113, 3). From the situation of the original point of evagination behind the tuberculum impar and vcntromesial to the two halves of the posterior segment of the tongue, the orifice of the pedicle corresponds to the line of junction of the three parts of the tongue. As a consequence, when these parts

Fig. 113.— DiagTammatip representation of pharynx of human embryo seen from in front (after Tonrnenx): I, II, first and second pharyngeal pouches ; 1, tuberculum irapar; 2, course of thy mglossal duct leading from 3, median lobe of thyroid gland; •I. laryngotracheal tube: a, esophagus: 6, thymus: 7, epithelial body [parathyroid]; 8, lateral thyroid ; 9, postbranchial body [parathyroid ?].

unite, the pedicle or duct is prolonged upward and comes to open upon the surface of the tongue. The canal is known as the thyrogloBsal duct or canal of His. In the fifth week it begins to atrophy, and usually by the eighth week has become obliterated. Occasionally it persists throughout life. The foramen csecnm on the dorsum of the tongue is the vestige of the orifice of the duct. Other vestiges of the thyroglossal duct are sometimes present. For example, the lower part of the duct may persist as a short tube, the tlisrroid duct, leading upward from the median lobe to the hyoid bone; and again, according to His, isolated persistent segments of the duct constitute the little vesicles in the neighborhood of the hyoid bone which are known resjx?ctively as the accessory thyroid and the suprahyoid and prehyoid glands. According to some recent observations the lower piirt of what His calls the tliyroglossal duct gives rise ti> the pyramidal process of thn thyroid, whirh extends ujw waixl towiml the hyoiil bone, usually a litllc to the lefl of the mid-line. Tiiis impaired median anlage gives rise to tile isthmus of the adult oi^aii and abo, to a considerable jMirt at least, of eai-h lateral lobe.

The lateral thjrroida begin their development somewhat later than dues iho niodlan ]xirtion. In the embryo of 10 mm., the fourth inner visceral ftirroT or throat-noeket of each Bide pouches om to form a. vesicle {Fig. 113, S). As the vesitrle grows, its iH-dieli- W'TOmes attenuated and finally disappears. After their isolation from the thniat-jKwkets, the

Fio, lU.-Seml-dlaenmnuitlc iniutrallam (o i>how Ww iilUmnlu |>oaliliiii of (be ttifiniu, Ih^rold gUntl, *nd pcMibranoliUl txid]' on thu niTk of tlii' vlilpk {A) and the K«1f(B). niter de Mciimn: ■■(, Ihjrfiid Elnnd ; p, postbranrhlal bodjr; «. Ihymxa: r, epithelliltiody [pumlhjniW]; ;r,lrBcbBfl; A.hrort; t^, vena Jugulmrls i eo. eiRHid Ti-ln.

vesicles give out small bud-like processes after the usual manner of the development tif glantl- and gradually approach the median lobe {Fig. 1 14, B), fusing with its posterior surface. The three parts unite probably in the seventh week. In the vertebrates below mammals the lateral parts of the thvmid darated from it as the suprapericardial bodies. According to the older view of His, the lateral thyroids produce all of the lateral lobes of the adult thvroid; laler i-eBoarehes have shown that they do not, but

THYROID, PABATHYROIDy AND THYMUS BODIES. 229

authorities are not in harmony as to whether they produce a large part or only a small portion of the adult lateral lobes. The more recent view of His is that the adult lateral lobes develop only in part from the lateral anlages. Verdun, the most recent worker in this field, maintains that the entire thyroid body of mammals and man is developed from the median anlage and that the podbranchial bodies (Figs. 113 and 114), by which name he designates the structures referred to above as the lateral thyroids, atrophy.

After the union of the three portions of the gland, the latter consists of a network of cords of cells, the meshes of which reticulum are occupied by embryonal connective tissue. Subsequently the cords of cells become hollowed out and exhibit alternating enlargements and constrictions. By the increase of the constrictions the continuity of the cell-cords is interrupted at short intervals, and so the network is converted into numerous closed follicles lined with epithelium, the formation of follicles beginning in the eighth week. The follicles later undergo considerable increase in size on account of the secretion by their epithelial cells of a peculiar colloid material, characteristic of the thyroid body. The embryonal comiective tissue, made up necessarily of mesodermic elements, furnishes the comiective-tissue framework and the blood-vessels of the organ, while the epithelium originates in the manner indicated from the entoderm of the gut-tract.

The Parathyroid Bodies. — The parathyroid bodies, usually two in number on each side, were discovered by Sandstrom in 1880. The lower pair lie upon the trachea in close relation with the thyroid body, while the upper pair lie at the level of the lower border of the cricoid cartilage, in relation with the dorsal surface of the lateral lobes of the thyroid body.

Their origin is still somewhat obscure. Apparently they are outpouchings respectively from the third and fourth visceral furrows, being composed, therefore, of entodermal epithelium. These epithelial bodies develop in a manner similar to the development of the thyroid body, but the fact that the cell-gi'oups are not broken up by the invading embryo anal connfdive timue to the «(me extent as in the ease of the thyroid rendei'a them hixlolagicaUy distinfjuhhabh from the laltrr ; moreover, it is stated by Maiirer that they never form colloid Hiibstjiiicc.

The Thymus. — What remains of the thymus after the second year of life is in;i(io up chiefly of Ijrmplioid and connective tissue, (.'nilniiilrd in wliicli are c ha racl eristic little epithelial bodies, the corpascles of Hasaall.

the epithelial parts of tlic thymns, in all vertebrate aniinalij, are derived from the entodennal lining if the pharyngeal region of the p;ut-trart. In tiio lower gronps, such as reptiles, amphibians, and bony fishes, the epithelinni of all the inner visceral clefts or throat-pouches shares in the development; while in birds, only two or three clefts bike part. In mammals, however, including man, the thymus body is derived probably from but one throat-pocket, the third.

The entodennal epithelium of the third inner pouch becomes evRginated (Fig. 113) to form an epithelial sac whose connection with the pharyngeal cavity is subsequently lost. The isolated and elongateil sac soon gives out small lateral buds or processes at the distal extremity. While the original sac has from the first a cavity, the bud-like branches are solid masses of epithelinm. The liranching continues and affecl^ not only the lower or distal extremity of the thymus sac but also the proximal end, the structure now resembling an acinous gland (Fig. 115). While this growth is taking place, the epithelial muss is being invaded by lymphocytes and young conneetive tissue with developing blood-vessels. (According to some recent studies by E. T. Bell, the lymphocytes are dcrivetl from the epithelium of the originul Ihymna anlage; but this is denied byStohr.) The encroachment by these elements continues to such an extent that lymphoid tissno — including leukocytes and ervthroblasts — becomes the predominant constitnent of the thymus, the epithelial parts sutferitig rc<luction, relatively, and liecoming fiiiiilly broken up into isolated maswa which are the corptucleB of Hassall of the mature gland. The breaking down of the epithelial cords is probably res[>onsible also for the irregular cavities of the thymus. Not until after birth do the glands of the two sides of the hcKly unite to form a single vnpaiied strnctore, and the development of the thymus is not completed until tiie end of the second yi'ar of life. Having attained its full development, the organ l>egins to retrograde, and ut the time of puberty has almost disappeared. Although sometimes persistent throughout life, it is usually represented by an insigniticant vest^ ige. (It has recentlybeen said that the thymus increast^s in size and weight up to puberty, and that it is an active organ until the fortieth year, after which time it atrophies.) While the epitheli&I parts of the thymus body, represented in the fully developed oi^n by the corpuscles of Hassali, are derived from the entodennal epithelium of the third inner visceral furrow, all other parts, the Irmpboid tisBoe, conoectlTe tissue, and bloodvessels, are products of the surrounding mesodenn.

Fni. lis.— ThyiDua of ao embrro nbblt or ■liteen da;B taRer K&Ulker), magnlfled: a, canal of the tb^mus; b, npper, c, lower end of Ihe

CHAPTER XIII. IIIIJ UUVRLOPMENT OF THE GENITO-URINARY SYSTEM.

OwiNU to tlio intimate anatomical and functional associa\{\\\\ \\( llio ^viiomlivo organs with the urinaiy apparatus, it U M^HH^*iM'v to iliseuHH the development of these two systems

MU'« lUiVlilOPMUNT OF THE KIDNEY AND URETER.

V\\y \*M^iu ol* {\\v ki(hu'y and ureter of the higher vcrte^^^^U^-^ U rtwn^oiat^^d with the development of certain fetal ^»V^M\-UM\'^^ \\\\' VHW^jfiiTOU and the mesonephros, which represent Sv |Hv ^uvl\ \\\s' kidiiry t»f hirval amphibians and the perma\\\M\ ki\\\\\\\ \»r llfthrw. In man and other allied types, the U^ui^ 4^^^^^^o i« of litthM)r no importance functionally, \\luK ihv' Iwwv t\iiu»lioimteH (hiring a i)art of fetal life as the '«»<.. ^^ \a uuu.M\ *'\oi'vti(»n, prior to the development of the |i\Muu^ui lvidiu\\.

\\u ^uvu^i^k^vui \»r ht»A(l kidney constitutes the most primiVvw ^I'U^UiAW I.VM^ pr)unc(*hiinism for the excretion of urine, iln iui«(\iir mii',mi*Uvi iu the foUowing manner: When Ok pu.ivtil iiu .mUiiu, Nshioh Hnbsc<jncntly divides into the

MMU, \ .\\iM\\\ (o riAjuMuto Ironi the lateral plate of mesoU i»u \Ux i\\,i |tvit t lU'o oouneoted for a time by an interven(M. )m.( I .>( K air, ibc middU plate or intermediate cell-mass \U. u». I hi ihii'kndiig of this intermediate cell-mass y\ ' l»i . 1 1*. Wu^UiiiU iidgt». whioh projects into the ctelom or I- ■ U M'iv ri»*' uu'^iuU'iiual i»r mesenchymal eh»ments of I'l \\ i'Uli in udm* brroiiu^ i^i'ouprd into cords of cells which it ui iiu. I liMU .u \'i'ii;iiu points with the mesothelial cells

I \U ■ s liMii I hr iU'iv'iu of (hcs(« et»ll-cords of the Wolffian \.A^,. \i\ hiiij^; biiu u Uk;Uici' ot' dispute, some authorities

maiiituiniiig that they come from the mesothelium of the body-wivity, while others believe that they are of ectotlerraic origin. Further cimiiges bring about the hollowing out of the cell-cords so that there results a long tube, the pronephtla or Besmental duct, which has several siiort tranaverre tnbtilen — ill some vertebrates, sis ; in man, two — ojwiiing into it ami communicating by their opposite oi>cn extremities, the nephridial fonnels or nephrostomata, with the cojlom (Fig. 117). In the human embryo the pronephric tubules have been found with oi»cn nephridial funnels, but without connection with the pronephric duct. The mesothelium in imnifxliato proximity to the open end of each short tubule is invaginated

Axia/trnr. , IflanU canml.

'1^.

Lttttal fUuti/sr

by a tuft of capillary blood-vessels from the adjacent primitive aortro to constitute a glomemlna (Fig. 117, hh). The pronephric duct passes tailward and opens into the cloaca,- a receptacle which receives, in common, the terminal orifice of the primitive bladder and that of the primitive intestine. It is apparent, therefore, that the pronephros or head-kidney is anatomically adapted to the function of removing certain substances from the blood by virtue of the action of the cells surrounding the glomeruli or tufta of capillary blood-vessels, and that these substances may be conveyed away through the duct into the cloaca and thence evacuated from the body. This organ is functionally active, however, only in certain lower classes of vertebrates, as in the Amphibia during the larval stage and in bony fishes. In mammals it is exceedingly rudimentary and very soon gives place to a more important organ, the mesonepliros.

The MesonepliroB or Wolffian Body. — As in the case of the pronephros, the origin of the mesonephros is to be found in the Wolffian ridge. Reference has been made, in treating of the primitive segments, page 77, to the middle plate (Fig. 116) as a tract of mesodermic tissue connecting the paraxial tract with the parietal plate. When the paraxial mesoderm

Fk;. llT.^Diagnim of pronephros (F) and proncphric duct (Prf): Al, allantols; G, gut; a, cloaca; W>, glomeruli.

Fig. 118.— Diagram of Wolffian body and duct : AU allantols ; (?. gut ; (jy, cloaca ; K. kidney evagination.

segments to form the somites the middle plate likewise undergoes segmetation, each segment being designated a nephrotome. Each nephrotomo, in the lower vertebrates, contains a cavity which communicates with the general bodycavity and which is, therefore, in effect, an evagination of the mesotheliuni of this space. In mammals, however, as well as in reptiles and binls, the nephrotome is a solid cord of cells. I5y the hollowing out of these cell-cords or nephrotomes a scries of transversely directed tubules is formed, each nephrotome, in fact, becoming converted into a short

Fig, 119.— TmiiJivcrso •cction of ■evenWen-day sheep ombryo iBonnetl; n«, ■nnlnn: on, amniotic atic-. n. nearal canal; >. ■omlte dUTerenllatecl Into miucleplate: H'd, Walfflau dut^l: R'b, Wulffliiii body: pm. psHvCal meioderm; vm. visceral lueaadenn; a, a, tUiIng primlllre aortas ; I, InlcsMne.

canal. These tubes acquire oonnection by their deeper ends with the previously formed proncphric duct (Fig. ] 1 7), which

Fro. lai.-DtepnaUlo

1/'

or Che M

,;:?*2i^-Jy

"^

6.6 cm (12 In.l long (To

, Wolffian body :

^ m-ary : a. InBUlnal llgamtnl:

• , diBphrBpn^llo IlKBine

nl:5,«OTi

ach ; 0. tnwstinp

7, bU.lder: s. unibilitiil Httery.

is known hereafter, therefore, as the mesonephric or Wolffian duct (Fig. 118). The latter duct and the short transveraa

liibulow which open into it constitute the Wolffian body or meaoaephros (Figs. 119aiid 120, 1). the tissue of the intermediate eell-nias!) from whicii the W'ollflan tiihiil<'s develop is designated, by Sedgewick. the Wolffian blastema, and by Rabl and by Schreiner, the nephrogenic tissue. At tliii* stage of its development the Wolffian bmiy consists of a tube or duct lying behind the parietal layer of the mesoderm, parallel with, and lateral to, the primitive vertebrid column, and opening at the caudal end of the embrj-o into the cloaca ; and of a series of transverse Wolffian tubules opening into the duct and abutting by their opposite ends upon the bodycavity. At the bead-end of the Wolffian duct the now atrophic pronephric tubes are still in connection with it.

As a farther step in the development of an organ adapted to the function of the secretion of urine, each Wolffian tubule becomes somewhat saccular midway between its two extremities, and this dilated ptirt of the tubule is invaginated by the capillary branches of an artery from the aorta. In this manner the cells that line the tubules are brought into relation with the bltK)d of the fetns and acquire at the same time the charaotors of secreting epithelinra. Such an invaginating tuft of capillaries, known as a glomernlns, with its enveloping capsule of Bowman, which latter is the invaginated saccular part of the tubule, constitutes a primitive Malpighian cor* puBcle, a ritructure analogous to the Malpigiiian corpuscle of ihe permanent kidney. Thi.t simple form of the mesonephros is seen as a permanent structure only in some of the lowest vertebrates. In all higher vertebrates it attains to a more complex degree of development, reaching its maximum in man in the seventh week of fetal lii'c. Its complexity ia increased by the dcvelopmont of secondary tubules and Malpighian corpuscles conneoled with those first forme<l. White at first the number of tubules corresponds with the number of nephnitomes, tlus corre^^pondenee is soon lost by the appearance of the secondary tubules.

The horizontal or transverse tnbnleB of (be Wolffian body arc divisible into an anterior or iipf)or scries, diatiiiguislied us the sexual aegmeut, anil a lowir set of atrophic tubules — atrophic for reasons that will appear hereafter. In certain vertebrates that are of higher type than those in which the pronephros functionates, such as adult amphibians and fishes, the Wolffian body persists throughout life as an organ of urinary secretion. In binls and mammals, however, its functional activity is but temporary, since it is supplanted, before the end of fetal life, by the permanent kidney. In man it disappears relatively early, retrogression beginning in the eighth week and the Malpighian bodies having almost disappeared by the fifth month. The presence of the mesonephros as a temporarily functionating organ in birds and mammals, while it is a permanent structure in certain lower members of the vertebrate series, exemplifies the embryological principle elsewhere referred to, that the higher types pass through stages during their development that are permanent in some of the forms below them in the scale of evolution.

The Metanephros or Permanent Kidney. — While the Wolffian body is temporarily functionating as a kidney, a structure is developing from the lower, caudal end of the AVolffian duct which is to form the permanent organ. It has been stated that the Wolffian duct opens into the cloaca. From the dorsal asjiect of this duct, near its cloacal end, a small diverticulum, the kidney evagination (Fig. 1 1 8 and Plate VII., 1), grows forth and soon lengthens into a tube which grows head ward, dorsomesial to the Wolffian duct, penetrating into the nephrogenic tissue or mesonephric blastema (p. 236).

The cephalic end of the tube dilates somewhat to form the primary renal pelvis, the anlage of the adult pelvis of the kidney, while the duct itself becomes in time the ureter. From the primary renal pelvis several small divertictila pouch out (Fig. 121), while the surrounding blastema becomes condensed and vascular.

Fig. 121.— Diagram to show extension and branching of kidney evagination and separation of its stalk from the Wolffian duct: u,primitiYe ureter; ;>, pelvis of ureter; WD, Wolffian duct ; Bl^ bladder ; u*, urogenital sinus ; CI, cloaca ; G, gut.

The development of the kidney from this stage onward has been for some years a disputed question. According to the older conception, still maintained by Golgi and by Minot, the small tubes which branch from the primary renal pelvis become the collecting tubules of the kidney and themselves give off branches which, increasing in length and acquiring tortuosity, become the secreting tubules (proximal and distal convoluted tubules, loops of Henle, etc.). The blind end of each convoluted tubule, becoming dilated and saccular, is invaginated by a tuft of capillary blood-vessels, thus being converted into a capsule of Bowman. The invaginating mass of blood-vessels constitutes a glomerulus, and

Mesodermic tissue.

i Wter.

V*.. V > \Mi^v<«m^^MMo n>)xiriiontAtton of the development of the kidney (after

Ui'Kvubaur).

^KiiMs'MiUu ausi s^i|wulo of Bowman together make up a llH^VA^t^UH sSMl^UiK^Wx Thus the entire system of tubules i^y K I hi I w \\\\ \\w i^lvin aiul the ureter have a common origin lU'iu \\u^ ^A\\^^\\ \'\\\\ \»t* thv Woltlian duct, while the blood\v V I \\\x\ Ks^wwwMW' {\^<\\K\ as well as the capsule, originate u villi ()i. uiuMtiubu^ Huvvnvhymo.

\. V I'lvliu^' lo \\\\^ K^\\wy viow — S»mi>er, Sedgewick, Balfour, »u.l ill. I \\\\\\\ iasUHiiiuhI bv the n»searches of Schreiner, wU- \y nil. li»\*' tnvh v^mtlmuHl for the most jwirt by UnJ». I. »li. « ^uixmIiu^hI uibuKvH and the capsule of Bowman ..ULUint ux.i \ *\U II .u^h.'t v»t* ilivertieula from the primitive \K\\.\\ |ul\i . tiui iiuU^ik^ikU^uIv tViMU the nephrogenic tissue,

and in the folluwiag inanoer : The nephrogenic tissue into which the kidney evagination peaetrates shows a differentiation into two zones — an mTtefOfif,immediatelysurrounding the primitive renal jielvisij consisting of epithelioid cells, and an oiUer zone of less differentiated mesenchyme. Soon after the appearance of the small diverticula which evaginate from the primitive renal pelvis and which arc designated the primary collecting tubules (which lattercorresjxtnd with the " primitive renal vesicles" of Haycraft), the nephrogenic tissue breaks up into smaller cell-masses, each snch mass surrounding a primary collecting tubule (Fig. 123, mk). This part of the nephro

FiO. 1Z3.— Section throuRh the kldn«]r tiT human TeIus <if Eeven monlhs (from Felii. sfterSchreinert: Sr, collt-cllnu tubules of wlilrh three nre ehuwn. each with itfl cap of metauepbTogenlc tiHue, mt; in relation with each is un early UTiniferoua tubule, the three latler. n, b, c, cuch nbovlnif a dlHerent itBgc nf development— a, showing beglnnltiK of eipanslon; b, evaclnatlon at At; r, S-sbapeil aiage, M Indicating development of Bouniana capsule.

genie tissue Schreiner calls the metanephrogenic tiasoe by way of distinction from the remaining part of this mesenchymal aggregation which, from its relation to the development of the mesonephros, is called the meBonephiogenic tissue. Each primary collecting tubule, after becoming bulbous at its end, divides into two tubules, each one of whicli in ttirn divides into two, this process of division Iwing repeated several times. These tubules become the adult straight collectinK taboles. The branching of (be primary collecting tubules continue'^ to the time of birth ; or until the fifth fetal month, according to Hamburger.

In the development of the secreting tubnles the inner zone of nephrogenic or metanephrogenio tissue alone is concerned.

This tissue preseDts little btid-like prulongatioiis, each such little bud of i-ells later acquiring u luiuen and M'jurating from the pureiit tissue (Fig. 12^,1, 6,c}. The buds are now small sacs, the renal veaicleB (Emery), of which there are at least two for each collecting tubule. Each vesicle now elongates and assumes an S-shaped form, the concavity of the up]»er part of the looking toward the collecting tubule, the vesicle on the right of the tnliule being, therefore, a reverse.1 S (Fig. 124). The lower limb of the^ S, from being simply tubular, becomes expanded, its upper wall being indented (Fig. 124, x), no Ihat it acquires the shape of a double- 1 aye red saucer, the space between its two layers being continuone with the remaining part of the lumen of tiie S tube. In the concavity of the saucer, beneath the middle piece

of the S, a strand of mesenchyme makes its appcamnce and into this tissue blooil-veKscls penetrate, hi that it finally becomes the glomemlns of the Malpighian corpuscle. The saucer-shaiied lower limb of the S becomes the capsnl* of Bowmaa; the lumen of the saucer, the apace of fiowm&n. Meanwhilii the upjwr limb of the S acquires continuity with the collecting tubule in close relation with which it has developed, and the two extremes of the S l)eing thus relatively fixed points, the ensuing elongation of the intervening portion neces^ii tales the formation of curvatures. A small part of the h)wer limb of the S, not being concerned in the formation of the saucer-shnjied anlage of Bowman's capsule, Wcomes a jmrl of the proximal convoluted tubule, the remaining portion of the latter, and tlic suc<vt'ding loop of Henle, the distal convoluted tubule anil the arched collecting tubule, being develojied respectively from the succeeding parts of the S. The outer zone of the metanephrogenic tissue gives rise to the capsule of the kidney and the supporting connective tissue, ineludiDg the columns of Bertini.

Iftnm Felix, after Bloofkl; Jir. ■mpulla af cnllvcUoR tubule ; oB. upper limb nf S; hB, lower IJmb of S (Bowmmn's cap(Die); X, pmllinn occupied by glomeru

The kidney acquires its characteristic features by the end of the second month of fetal life, and it reaches its permanent position by the third month.

THE SUPRARENAL BODIES.

The development of these structures has been the subject of much discussion. It has been maintained, on the one hand, that the cortex of the organ develops either directlv or indirectlv from the mesothelium of the body-cavity and that the medulla has its origin in outgrowths from the sympathetic ganglia ; on the other hand, that the entire organ is a product of the mesenchyme — indirectly, therefore, of the mesothelium. Thus Minot, Human Embryology, 1892, remarks, "That both the cortex and the medulla of the adult organ are formed in man from the mesenchymal cells, as Gottschau showed was the case in several mammals, is, I think, beyond question "; but in his Laboratory Text-book of Embryology, 1902, p. 267, the same authority, in describing pig-embryos, says : "The sympathetic tissue gives rise to the so-called medulla of the adult organ." Again, Aichel, 1900, from his investigations concluded that both medulla and cortex arose from the mesenchyme. O. Hertwig, in his Lehrbiich, 1906, expresses his conviction of the correctness of Poll's ^ conclusions as to the double origin of the organ.

The cortex, according to Poll, whose work reaffirms in many particulars the conclusions of some earlier investigators, arises from small bud-like masses of cells that come from the mesothelium of the coelom in close relation with the genital gland and the mesonephros, but distinct from them. These buds lose their connection with the coelomic epithelium by the fifteenth day in the chick. They are situated on each side of the root of the dorsal mesentery and all those of one side unite to form a single organ. This occurs in man by the twenty-eighth day (Souli6). In the lower vertebrates this organ fails to unite with the anlage which represents the medulla of the higher vertebrate suprarenal body and constitutes the separate interrenal organ of the lower vertebrates. In some cases — e. g., in sharks — the anlages of the two sides unite with each other, forming a single unpaired interrenal organ, which lies l)etween the primitive kidneys. Failure of union of some of the buds, it is believed, is responsible for the accessory suprarenal organs of Marchand which are occasionally found between the layers of the broad ligament of the female or in relation with the epididymis of the male> these having followed the descent of the testis or ovary respectively.

• H. Poll, Die Entwicklung der Nebennieren Sjrsteme, Handbuch der

vergleich. und experim. Entwicklungslehre d. Wirbeltiere, Bd. III., Abt 1, 1905.

The medulla of the organ, still following Poll's account, originates at a later stage than the cortical anlage from chains of cells that grow forth from the sympathetic ganglia and form groups which for a time retain their connection with the ganglia. The cells show a differentiation into two classes, the sympathoblasts and tiie phjeochromoblasts, the latter gradually becoming the phseochronie cells, so called from their staining darkly by chromium salts. In many vertebrates these cell-masses remain separate from the interrenal anlage and constitute the phseochronie bodies or suprarenal bodies of lower vertebrates. In birds and rej)tiles the union of the phfeochrome and interrenal anlages occurs by a mutual intergrowth of their cells, the result being an irregularly stratified organ ; in mammals and man, however, the cells of the sympathetic anlage gradually {wnetrate in the form of cell-coixls into the interior of the interrenal anlage to occupy their adult position as the medulla of the organ. This process of intergrowth continues in man until the time of birth.

In the early stages of fetal life the suprarenal body is relatively much larger than in the adult condition, and is situated chiefly on the ventral surface of the kidnev. At about the thinl month it Iwgins to assume more nearly its normal position.

The account of the development of the bladder and of the urethra mav be deferred until the evolution of the internal sexual system shall have been considered.

THE INTERNAL GENERATIVE ORGANS. 243

THE DEVELOPMENT OF THE INTERNAL GENERATIVE ORGANS.

The Indifferent Type. — The internal generative organs of both sexes, in the course of their development, pass through a stage in which there is to be found no distinction of sex. This stage is designated, therefore, the indifferent type of sexual apparatus.

While the Wolffian body is attaining its full development, there appears in its vicinity a tube, the duct of MUUer (Plate VII., Fig. 1), which lies parallel with, and to the outer side of, the Wolffian duct. In non-amniotic vertebrates the duct of Miiller arises by fission or longitudinal division of the mesonephric duct. Its exact mode of origin in the amniotic vertebrates is not as yet definitely settled. According to one view its upper or cephalic portion is produced by an evagination of the mesothelium of the bwly-cavity, while the remaining lower segment results from fission of the mesonephric or Wolffian duct. According to another view the lower or caudal portion is produced by the direct extension of the upper portion in the caudal direction by the proliferation of its own cells. In whatever way the duct may be formed, its lower or caudal end opens into the cloaca, which receptacle receives also the termination of the Wolffian duct. The upper end of the duct maintains a communication with the body-cavity or coelom by means of an expanded funnelshaped mouth. Its lower segment is closely associated with its fellow and with the Wolffian ducts, forming thus the genital cord. The function of this canal in lowly organized animals — that of receiving from the body-cavity the female genital products, the ova, and evacuating them from the body — foreshadows its subsequent metamorphosis in most vertebrates.

While the duct of Mfiller is forming, the mesothelial cells overlying that part of the free surface of the Wolffian body which looks toward the median plane and somewhat forward, its ventro-mesial aspect, undergo multiplication and thickening (Fig. 125, a), forming an elongated swelling or ridge. This is known as the genital ridge, which produces a projection upon the wall of the body-cavity. The genital ridge is still further thickened by the proliferation of

llio niPstKlermid tissue (£') beneath the niosothelial cells. The genital rulgos ol' the liiimaii fetus apijesir in the fifth week.

Further differentiation of the genital ridge results in ita tninRformatiun into the so-cilUciI indiffeient sexual gl&nd (Plate VII., Fig. 1), a structure common to Iwth sexe.s at this stage. The essential feature of this process is that the thickened raesothelial cells overlying the genital ridge become modified in character and penetrate the ridge in the form of conls or Btrands of cells. These mesothelial e]eioent.s were called by

Fin. 1».— Crou-fcrtlnn Uirnugh IcrlBn duct, and the leiual gland of vMcV nr i mrunlnud 100 dliiineMn: m, iDeacDti>ry: L, Mimatopleun germinal epllheliiim tnim wlilob the Milllprlan dtict (i) hi Uilckcncd pari of the Rcrminal eplih.lium, iii wlilch the

r the UUlilsy (afttr WaldcyOT), a', [he nglon of the been invaglnated ; a, rlRiary sexual cvlli. C

Waldeyer the genninal epithelium, because, after their extenaion into the interior of the ridge or gland, they give ri.se to the genn-cells, namely, the ova or the spermatozoa as the case may lie. The eelt-cnrda include two kinds of elements, the smaller mesothelial cells and the primitive sexual cells, which latter art! laryir und less numerous than the mesothelial cells

I>ia?ramm«llc reprewnliillnn of the ilcmlopmeiil of [be genllim WolIBiMi bod; and Its derlvittlTeg belni; colored red, the MUUerlBn rtTBtlves, green: I. ludHRri-nt type; i ilnHBerent type, Islet Btage, HfllleTlan ducta and tlic piimillve areter now openlog into ibe iirc^eultal ilnn* e type, lower ooda of MaUerlMi dnole ftued lu futm Ibe ilniu pocuUirli: ~

B type.

Xom, the

.,»«,. I

nkn and ^M

awl kive hr^ niicK^»lati*tl iiurl<M. Tlir tirlmlHv«i m^^^At

or primitiye ota, tin' mi imiIIihI iM^Hiiiat' H Ihi that thoy ilovolop olllirr inli» \\u' n\ii tit 0\t* 1 filaments accimliii^ tt) \\\v **v\ nl iIh' i'ImI«i\i« t'lu* ceU-cords have boon so<mi in \\\v iiHlillrrtnl iiImmiI i«I tltr hmun embn'o a:!: oarlv as (In* lil'tli wv^U \\ \\s\^ \\\s\\ althoagfa thero are no ^ross st^xiiitl tliiliiiiliitn i ii t>i«iMti"*ti«ii it ia po^iblo to ilotonninr iVnni tlir lii^tolnpitiil i)fn*ti'ii \ M the organ whether it is tn Im* n Itwti^j m tiM«M*n\. \\\* l'M«it> oells bein^ far loss iniinrniiM itliiti\i>l\ in \\\* t^Mni t

se than in the hittor (Nii^rl).

The indiBbront soxnal ^IiiihI iMiinr : imIh 'Hi i-^pn i'lllx .In .• rehitiun with tho n|)|NT or si*\niil •:i'iii> nl tin- nii i^nipln* orWolffianb<Kly (IMato V 1 1., \'"\\\. 1 1. llM«'i|»niMi -Mh . .m \\!\\. U fact will a|>|)oar hit or.

The Male T3rpe of Hnminl FtriHiMM

The teSticlo llil-: IL <ImiiIi|i. Ill ik:iM ini. llii |«tiipti qiii'titiotV part of tho orpin i -. iniiiliniil In dti im ('ihiiii|ilin t - •«! ilio

indifToroiit scxiimI ^'IiiimI. \Jiili ii - :\ dm ni pfYptonl iturU i-i furnisli(;(I liv ilw Wnllli'in Imih Miiifmn h'l > liri-n nciilr

■

of tho O4;ll-r.onl"- and tho lar^^rr and Irrvi ninnrroiiM priinitivo srvnal (*olls. The mesothelial coIIn inrrraso in nninhor and Uooonio so grouped as to form (^ylindrioal masses known as sexual cords, each of which inolndos sonu* of tlu* primitive seminal or sexual cells. By the ingrowth of connective tissue from the surrounding mesoderm, the timica albnginea is formed and the sexual cords are divided into roundish masses, each of whicli is made up of many of the smaller elements and a less number of the large seminal cells. These follicle-like masses become hollowed out to form the seminal ampulla, which afterward, undergoing great increase in length, are transformed into the seminiferous tubules. During fetal life, however, and even to the period of puberty the "tubules" remain solid cords of cells. The small mesothelial or epithelial cells give rise to Sertolli's columns, while the primitive seminal cells produce the spermatogonia.

Spermatogenesis, or the development of the spermatozoa from the cells thatline the seminiferous tubules of the functionating testicle, has been considered in Chapter I.

While the sexual cords are bein": transformed into the cylinders that become the seminiferous tubules, the surrounding mesodermic tissue ix^netrates the genital gland and forms the connective-tissue septa that constitute the stroma of the organ and divide it into lobules. At the same time, also, marked chanj^es occur in the Wolffian bodv. From certain of the Malpighian corpuscles of this structure, cords of cells, the medullary cords, grow forth and penetrate the genital gland, their ends fusing with the primitive seminiferous tubules. The conversion of these cell-cords into tubes furnishes the initial part of the system of excretory ducts of the testicle, namely, the vasa recta and the rete testis. Somewliat later, in the twelfth week, the rete testis is extended to form the vasa efferentia, and still later, in the fourth and fifth months, the efferent vessels lengthen and become tortuous, producing thereby the coni vasculosi or head of the epididymis (Plate YII., Fig. 3; Fig. 126). The upper part of the Wolffian duct develops into a convoluted tube which constitutes the body and tail of the epididymis, while the lower |X)rtion becomes the vas deferens, thus completing the system of canals provided for the escajx* of the spermatozoa. Near the caudal end of the Wolffian duct a little pouch-like evagination grows from its wall and becomes

Fia. 126— Inlemal generative o EBni of ■ m&le fetua of about fouruc wiiekB (Waldeyer); (. testicle; t, ej dldymla: f. Wolffian duct: u>. lowi jlfflan body; j, guberoac

the seminal vesicle, the lower end of the duct, below the orifice of the semiDal vesiule, being the sjacnlatory duct. Since the "Wolffian duct terminates in the cloaca, and since the anterior part of tbc cloaca corresponds to a portion of the later urethra, the termination of the ejaculatory duct in the prostatic part of the urethra ig accounted for. Thus it will be seen that while the secreting part of the testicle results from the transformation of the indifferent genital gland, the secretorr cells having their origin in the germinal epitheliam, the complicated system

of dnctd with which it is provided is fumbhed by the mesonephros or Wolffian body.

The series of tubules connected with the upper extremity of the Wolffian duct, the remnant of the pnmspbros or headkidney, frequently persists as a little pedunculated sac attached to the upper part of the epididymis ; it is known as the stalked liydatid and sometimes also as the hydatid of MorgagnL The posterior or lower set of Wolffian tubules likewise give rise to an atrophic structure, the paradidymis or organ of Oirald^a, which consists of a series of short tubes closed at both ends, lying among the convolutions of the tail of the adult epididymis (Plate VII., Fig, 3), while a lateral evaginalion from that i>art of the Wolffian duct which forms the tail of the epididymis becomes thevaa abeirans.

The dnct of Uttller remains atrophic, in the male, throughout its entire extent, and in fact, with the exception of its two extremities, it usually altt^ther disappears. Its upper extremity persists as a small vesicle, the nnstalked or sessile hydatid, attached to the upper aspect of the testicle. The lower extremity of the duct, uniting with its fellow, becomes converted into the sinus pocnlarls or ntenu mascnlintiB of the prostate gland (Plate VII., Fig. 3). If the intervening part of the tube persists to post-natal life and remains patulous^ it is known as the duct of Rathke.

the change of location whicii the testicle undergoes is a conspicuous feature of its development. To understand this clearly, it is necessary to recall the relation of the mesonephros and the genital gland to the peritoneum. Since both of these bodies originate from the cells of the outer wall of the body-cavity, or, in other words, from what becomes the parietal peritoneum, necessarily they lie between the body-wall and the parietal peritoneum — that is, behind the peritoneal cavity. With the increase in size of these structures, they project toward the peritoneal cavity, the peritoneum passing over them and forming a "mesentery," which anchors them to the posterior wall of the abdomen. In the case of the testicle, this peritoneal fold or " mesentery " is called the mesorchium ; in the case of the ovary, the mesovarium. It is prolonged upward to the diaphragm as the diaphragmatic ligament of the primitive kidney, and downward to the inguinal region as the inguinal ligament of the ])rimitive kidney (Fig. 120), since this latter organ is the largest constituent of the projecting mass. When the primitive kidney has disiippeared as such, the inguinal ligament nientiontul seems to connet^t the ovarv or testicle with the inguinal region of the abdominal wall.

The inguinal ligament eontiiins between its folds connective ti>«^ue and unstriped muscular fibers. These become the gub«»)iUACulum testis in the male or the round ligament of the uteiHH in tl»e female. As the bodv of the fetus continues to )i}\^\\ while the tissues of the ligament remain stationary or i;ro\N lr.-»-' rapidlv, the testicle is gradually displaced from its pmiiiioM al iho side of the lumbar spine, and by the third uioutii Uiuhr.-x I he false pelvis. In th(» fifth and sixth months it it in roui.u'l willi the anterior abdominal wall, near the iniu'i' iJuloioiiial ring. In th(» eighth month it enters the in>;ninal lanal, and \\\^\v the end of th(> ninth month, shortly bi-fou' Iniih, it K'a\e« the inguinal canal and enters the

• .\i>M «Uv.ivui .>i \\w Uviiis'U>M. \^llli (H>nstM)Ucnt emptiness and flabbinesH

Before the testicle leaves the abdominal cavity, the parietal peritoneum pouches through the inguinal canal into the scrotum, the protruded part being the processus vaginalis. Since the testicle is from the first behind the parietal peritoneum, it passes into the scrotum behind the vaginal process, the latter then folding around it as a shut sac of two layers. Subsequently the connection of the sac, now the tunica vaginaJis testis, with the abdominal peritoneum is reduced to a slender strand of tissue lying in front of the spermatic cord.^

The testicle necessarily carries witli it, in its descent, its blood-vessels, the spermatic artery and vein ; its duct, the vas deferens ; as well as its nerves and lymphatic vessels ; and these structures collectively constitute the spermatic cord.

The Female Type of Sexual System. — While the indifferent sexual gland, in the development of the male generative system, undergoes metamorphosis into the testicle, it becomes, in the evolution of the female type, so altered as to constitute the ovary ; and while the Wolffian tubules and the Wolffian body become in the male the system of excretory ducts of the testicle, they produce in the female merely atrophic structures. On the other hand, the duct of Miiller, which gives rise in the male, to atrophic appendages, forms in the female type 'the Fallopian tube and, by fusing with its fellow of the opposite side, the uterus and the vagina.

The ovaiy results from alterations in the structure of the genital gland analogous to those that occur in the evolution of the testicle. The special features of these changes are better understood, however, than arc those of the testicle. As in the case of the development of the testicle, the mesothelial cells on the peritoneal surface of the genital ridge become thickened, these altered cells constituting the germinal

of the scrotum, is designated cryptorchism (hidden testes). The presence of but one testicle in the scrotum is called monorchism,

• Occasionally it happens that the funicular process of the tunica vaginalis — that is, the stalk of the sac, remains patulous throughout its entire

extent, a condition which allows of the easy and sudden protrusion of a segment of the bowel into the cavity of the tunica vaginalis, constituting the so-called congenital hernia. Or the funicular process may close only at one or the other end, givitig rise to other varieties of hernia.

epithelium (Fig. 125). Coiuciden tally, (lie primitive connective tissue — iiiesodermic tii^ue — u ml e Hying the germinal epitheliiim pmliferates, contributing to the thickness of the genital ridge. By the sixth or seventh ^vcek, the germinal epithelium consi!^t»of several strata of cells, gruu{>3 of which begin to penetrate the umierlying niesodermio tissue in the form of cordlike procesBSB (Fig, 128, f, sch). The indifferent mcsodermic tissue at the same time increases in quantity, in turn penetrating between the groiijis of advancing cells, so that what takes place might be described as a mutual intergriiwth. The presence of the growing connective tissue acceniuatfis the grouping of the cells into cylindrical masses. These latter are the sexual cords or egg-colunma (Pfluger's egfi-tulie,-). They ccntyin twit special kinds of cells, the large sexual cells or primitive ova (Fig. 128, xie), and the smaller hnt more numenins mesothelial cells. The connection of the sexual cords with the germinal epithelium is much more obvious in this case than in the case of the developing testicle, and the primitive sexual cells are much more abundant. The e^-colnmns, surrounded by young connective tissue, constitute the nucleus of the cortical part of the future ovary. This mass is later sharply marked off from the free or peritoneal aspect of the gland, the region of the germinal epithelium, by a zone of proliferating mesodermic cells which become the tunica albuginea of the ovar^'. An important change now takes place in the egg-columns ; the primitive ova, or large sexual cells, increase in size, their nuclei l>ecoming espi-cially well ilevelojKH], while the small

Fig. 127.— Internal or^ni of ft remale Ibtiuof sbtiut fourtti'ii werks (WnMeyer): 0, ov»ry; *, epoSphoron or ptrovnriiim : u-.WulfflKD duct: m. MUllerlftD duct; v. iQwcr put uf (he Wulfflaii body.

mesothelial cells become smaller and less conspicuous. It frequently happens that several of the large cells fuse into a single mass of protoplasm, while one of the nuclei outstrips the others in growth and, with the surrounding zone of protoplasm, becomes the ovtun. Each egg-column is now broken up into several groups of cells by the penetration of connective tissue, each group (Fig. 128, c, «;/(') containing a single ovum, but many of the smaller cells.

Fig. 128— PBrt of safflltal aeclfon of ati iivarx of a ehlld jujt born (after W«ldeyor). Hlfdily matniifled: te, Berminal epilhi-llum ; t. acft. PflQger's egg-lubes; ue, primitive ova iylng on the germinal epilhclium ; e, kK, long PflOgert tubes. In process of being coiiverled Into folll«les ; el, b. L-gK-balla (nests), likewise in process of being resolved Into follicles: /, youngest follicle already Isolated: gu. bloodveBsels. In the tubes and egg-nests the primordial eggs are dlstingofshable from tbe smaller epithelial cells, the future follicular epltheUum.

These groups are the young Qraafian folllclas of the ovary (/). The enveloping zone of connective tissue becomes the tbeca of the fotliote, while the single large cell constitutes the omm, and the smaller cells are the membrana granulosa. At first the graniitosa cells surround the ovum as a single layer of flattened cells which gradually assume the columnar type and become so numerous as to form many layers. They secrete a fluid, the liquor folliciili, which crowds the ovum to one side of the follicle where it is enveloped by a special group of graniilosa-cells, the discns proUgenu (Fig. 129).

The question of the origin of the lolliouhir cells ia still an unsettled one, though it seenib probable, thut the> are derived from the cells of the egg <. Itimiis jnd Minot believes that thej are probabjv ikscendt i from the prmiitue o\a.

Till formation of new Qraaflan follicles iiid consequently of o\!i, begins in the decjitr pirt of thi o\ar\ and advances toward tlic iiurface TIk production ot o\a and follicles is

Fin. tiB,— Section of human OTary. including cortex . a, geriolnal epiltiellum or ft^e aurdicv: b, lunica Bltjuglnea; c, peripheral aironm canUlnlng loimBluTe Gnuflnii follicles Cf I -. c. WHtl-advonced fullicle IWim whcsc wall memljninil granaloBii hu partialis Mimrsleil :/. cavity of liquor folUcull: g, ovum surruunded by cell-miut coiistliullng illaciii proligenis (Pis reol).

limited to the fetal stage and to the early part of post-natal life, their formation not occurring, according to Waldeyer, after the second year.

What has been said above refers to the development of the cortex of the ovary. The medulla is produced by the growth toward the egg-columns of cord-like processes, the medollaiy cords, fi^m the epithelial walls of the Miilpighian corpuscles of the primitive kidney or Wolffian body, the cords l)e('oniing surrounded by connective tissue and forming a network. The fetal medullary cords are reprcsentwl in botli the cortex and the medulla of the mature ovary by the groups of interstitial cells di'-]>o«ed between the bundles of the stroma-tissue.

THE INTERNAL GENERATIVE ORGANS. 253

The Oviducts, the Uterus, the Vagina. — The system of passage-ways that constitute the outlets for the ova and the means of nourishing them and evacuating the product of gestation from the body in the event of impregnation — namely, the Fallopian tubes, the uterus, and the vagina — result from the metamorphosis of the ducts of Miiller. These ducts, as stated above, lie along the dorsal aspect of the body -cavity, separated from it by the parietal peritoneum, and parallel with the primitive spinal column (Plate VII.). The probable method of their formation has been pointed out (p. 243). Near the lower (caudal) end of the body they approach each other, and finally unite about the second month to form a single duct for the rest of their extent (Plate VII., Fig. 4). The upper, ununited parts of the ducts become the Fallopian tubes or oviducts, while the lower portions, now fused into one, become the uterus and the vagina. The upper end of each single duct expands trumpet-like to form the fimbriated extremity of the Fallopian tube.

Until the fifth month there is no distinction between the vagina and the uterus, the two being represented by a single sac-like structure. The development of a circular ridge in the wall of the sac marks the division between the two organs, the part above the ridge acquiring thick muscular walls, while the part below it, the future vagina, remains thin-walled and more capacious. In the third month the uterus is bifid at its upper extremity, a condition which is permanent in some animals and occasionally in the human subject.^

The Wolffian duct, which, in the male, becomes metamorphosed into a part of the epididymis and the vas deferens, remains undeveloped in the female, producing merely atrophic or vestigial structures (Plate VII., Fig. 4). The upper series of Wolflian tubules, the remnant of the pronephros, frequently

' The formation of the uterus and of the vagina by the coalescence of two parallel tubes affords an explanation of the uterus bicomis or bifid uterus and of the condition of double uterus sometimes met with, as also of the presence of a median septum in the vagina^ since by the failure of union of the two tubes in greater or less degree one or other of these anomalies would result persists, as in the male, in the form of a small pedunculated sac, the stalked hydatid or hydatid of Morgagni. When present, it is to be found in the broad ligament, in the neighborhood of the outer extremity of the ovary. The middle or sexnal series of the Wolffian tubules with the adjacent part of the Wolffian duct, which, in the male type, develop into the epididymis, become in the female, an atrophic structure known as the epodphoron or parovarium, or organ of Bosenmiiller (Fig. 127). This structure, which is almost constantly found between the layers of the broad ligament in close proximity to the ovary, consists of a larger horizontal tube representing a segment of the Wolffian duct, and of shorter vertical tubes joining this at a right angle and representing the transverse Wolffian tubules. The lower set of small Wolffian tubules, those which, in the male become the paradidymis, give rise in this case to a similar atrophic body, the parodphoron. This is also situated in the broad ligament, usually to the inner side of the ovary. The Wolffian duct, with the exception of that portion of it that assists in the formation of the parovarium, usually entirely disappears. Occasionally, however, it persists as a small canal traversing the broad ligament close to the uterus and passing on the dorsal side of the upper part of the vagina to be lost upon the wall of the latter or, more rarely, to open near the urinary meatus. When thus persistent, it is known as the duct of Gartner.

The change of position of the ovaries is similar to, though less marked than, that of the testes. The inguinal ligament in the female (Plate VII.) extends from the primitive position of the ovaries in the lumbar region of the abdominal cavity to the groin, where it pas>5es through the abdominal wall, traversing the inguinal canal, to terminate in the labium majus. The upper part of this ligament, coiitiiining involuntary muscular substance, firmly unites with the ovary. In the third month the ovary descends to the lower part of the abdominal cavity and is now connected, by the succeeding portion of the inguinal ligament, with the uterus. This connection may be a factor in the fiiial change of position of the ovary — that is, its descent into the true pelvis. The part of the inguinal ligament that passes from the ovary to the uterus is the permanent ligament of the ovary, ivhile the remaining portion, which passes from the uterus through the inguinal canal to the labium majus of the vulva, is the round ligament of the uterus. As the inguinal ligament perforates the abdominal wall, a small diverticulum of peritoneum goes with it. Normally this peritoneal pouch subsequently becomes obliterated. Occasionally, however, it persists and then constitutes the canal of Nuck. Should the canal of Nuck be present, the ovary may pass into or through it, thus reaching the labium majus. A patulous canal of Nuck, as in the case of a patulous funicular process of the tunica vaginalis of the male, may j)ermit the sudden occurrence of an inguinal hernia in the female. The mesovarium or " mesentery of the ovary accompanies the ovary in its descent and constitutes a fold of peritoneum which envelops not only the ovary but also the adjacent part of the duct of Miiller and the remnants of the Wolffian body. Upon the uniting of the lower parts of the Miillerian ducts to form the uterus and the vagina these mesovaria unite with each other mesially to become the broad ligament of the uterus. Thus it comes about that the uterus, the ovaries and their ligaments, the epoophoron, and other fetal remnants are contained between the layers of the broad ligament.

The account of the development of the external genital organs will be deferred until after the consideration of the formation of the urinary bladder and of that part of the urethra that originates from the same embryonic structure.

THE BLADDER AND THE PROSTATE QLAND.

As stated in Chapter V., the urinary bladder and a part of the urethra are derived from the intra-embryonic portion of the allantois. In the same chapter the allantois was described as a sac which develo|3ed as a pouching-out of the ventral wall of the gut-tract near its caudal end (Plate II., 5 and 6). The sac protrudes from the still widely open abdominal cavity, enters the extra-embryonic pAit of the bodv-cavit*-, and reaches the inner >arface nf the fidse amnion, with which structure it intimatelv unites to form the true chorion (Plate III.L A? tht- «-all> of the abd<4DeD gradually Q\i^\ leaving only the umbilical aperture, it is, neceRsarilyy through tlii< aperture that the allantois prxH trudes.

We have seen p. 91) what become? of the extra-abdominal part of the allantoic — in what dejrri'C* it contributes to the formation of the placenta and of the umbilical cord. Obviously, with the j^evering of the umbilical cord after birthy all this extra-embiyonic ]Kirt of the allantois disappearsy giving rise to no adult organ.

Its intra-enibiyonic ]K>rtion consists of a tube extending from the caudal end of the intestine to the umbilicus (Plate II., 5 and Gj. As early as the second month, the middle segment of this tube dilates and assumes the form of a spindleslia(KHl sac, which becomes the minary Uadder (Plate VII.). The part of the tube connecting the summit of this sac with the umbilicus remains small, gradually loses its limien, and constitutes in the adult the (usually) impervious cord known as the urachns. Should the cavity of the urachus persist in its entirety, and should there l>e at the same time an external o{>eniug at the umbilicus, the condition would constitute an imibilical minary fistula. The proximal {>art of the allantois — that is, the i)orti*)n intervening l>etween the bladder and the intestine — is designat(f<l the sinns nrogenitalis, while the caudal end of the intestine, which is, in (fffi'ct, a jH>nch in which lH)th the allantois and the intestine terminate, is known as the cloaca (Fig. 96). The urogenital sinus receives the terminations of both the Mullerian and the Wolffian ducts (Plate VII.).

In the sixth week or slightly earli<»r, there api)ears uix)n the surface of th(i IxhIv, in the region corres|M Hiding to the iK>siti(m of th<» <'loaca, a (l(>pr(»ssion, the cloacal depression (Fig. 96), which lat<T, except in man and the higher mammals, meets the cloaca, and thus establishes a communication between it and the exterior. In the Amphibia, in reptiles, and in birds, as also in the lowest mammals, the monotremes, the cloaca is a permanent structure, and through it, in these groups of animals, not only the fecal matters and the urine, but also the genital products, the spermatozoa and the ova, are evacuated from the body. In all mammals, however, with the exception of the monotremes, the cloaca undergoes division into a posterior part or anal canal and an anterior urogenital aperture. This division is brought about by the growth of three ridges or folds, of which one springs from each side of the cloaca and one from the point of union of the urogenital sinus and the intestine. These folds coalesce about the eighth * week to form a complete septum, which continues to thicken antero-posteriorly up to the time of birth and constitutes the perineum.

It will be remembered that the ureters originally spring from the terminal parts of the Wolffian or mesonephric ducts (Fig. 118), Owing to alterations brought about by processes of unequal growth, the orifices of the ureters subsequently change their position so as to open into the urogenital sinus (Fig. 121), and still later, by the further operation of the same agency, they come to open into the bladder on its dorsal wall, thus gradually assuming their permanent relations (PL VII.). After the division of the cloaca the urogenital sinus, as stated above, opens independently upon the surface of the body. In the female it is transformed into a short tube, the urethra, and an expanded terminal recess or fossa, the vestibule of the vulva (PI. VII.). In the male it becomes the first or prostatic part of the urethra.

In the twelfth or thirteenth week, the future prostatic urethra acquires very thick muscular walls, and the original epithelial tube pouches out into the muscular tissue in the form of little sacs, the lining cells of which assume the characters of secreting epithelium. In this way is produced the aggregation of muscular and glandular tissue known as the prostate gland. This is a well-developed structure by the fourth or fifth month (Tourneux). The recess in the floor of this part of the urethra, the sinus pocularis or uterus mas

• Fourteenth week, according to Miuot

enlintu, Iia« l>een previously referred to as the homolc^e of the atcrus, being the persirtent caudal extremities of the ducts of Miiller (Plate VII.).

THE EXTERNAL ORGANS OF QENERATION.

In the early stages of the development of the external f^nital oi^ns no ^xual distinotions are apparent.

Reference has been made to the eloacal depression as a superficial fossa which makes its appearance at the caudal end of the body of the embryo iu the sixth week (Fig. 96). At

Fig. 1:10.— Pi:ur iiut'i:vulvi' lUufoa titAe\e\n\iiaenltit tliv i;!!!'^!!!! gciiical organr (IndlR^rcnl typcl of Iho hurnun felui iif Sito M mm. {0.ie> to I J5 inuhl ITourneux) : 1, MEnlUl etnlneucG or lubcrck; 1, eliin<: 3. geoilal groorc; 4, gcnllal ridge; 5i

olilMBl ilupTWlxD : fi. cncr^Rcil emlaeace.

about the same period an ciieireling elevation, the genital ridge (Fig. 130, -"1,4), is seen to surround this depression. Within the gciiitul ridge, at the anterior part of the cloncal fbcsa, a email tubercle, the genital eminence, appears at the same time. On the under asjH'ct uf the tjonilal eminence there is soon distiiiguishftlile tin; genital groove (Fig. 130,3), which apjieare as if a continuation of the fis,sure-Iikc cloacsil dcpi-ession iji)^

THE EXTERNAL GROANS OF OENERATION, 259

and the groove very shortly becomes flanked by two ridges, the genital folds, one on each side.

The genital eminence becomes the penis or the clitoris^ according to the sex of the fetus. It very early acquires a knob-like extremity (2) which is the beginning of the glans penis or of the glans clitoridis, as the case may be. Further development of the glans is brought about by the appearance of a partially encircling gr(X)ve which serves to differentiate it from the body of the organ.

At this stage of development, the rudimentary organs, as described above, are precisely alike in the two sexes. Early in the third month — about the ninth week — sexual distinctions begin to become manifest. Since the female organs exhibit the less degree of deviation from the early indifferent form, they will be first considered.

The External Genital Organs of the Female. — The sexually indifferent genital eminence which, as we have seen, presents even by the end of the second month a rudimentary glans and an indicaton of a prepuce, elongates somewhat and becomes the clitoris. The genital folds bounding the genital groove on the under surface of the genital eminence (Figs. 130 and 131, A) never unite with each other as they do in the male, but become prolonged in the direction of the future anus and constitute, by the fourth month, the lateral boundaries of the orifice of the urogenital sinus, or, in other words, of the vestibule of the vagina (Fig. 131, 3). These folds, continuous over the dorsum of the clitoris with its rudimentary prepuce, are the nympha or labia minora (Fig. 131, 2), 7) of the fullyformed state. The masses of erectile tissue in close relation with each labium minus, the pars intermedialis and the bnlbns vestibuli, are the homologues respectively of a lateral half of the male corpus spongiosum and its bulb. The genital ridge, which, from the first, encircles the genital eminence and the cloacal depression, and, consequently, the later clitoris and the aperture of the sinus urogonitalis, increases greatly in thickness. The part of it situated on the ventral side of the clitoris becomes the mons veneris, while the lateral parts of the ridge become the labia majora of the vulva. The several jiarls of the female g«niJ talia develop lo such a degree durhig the fourth month tlmn their aexual characters at this time are well marked.

Fig, ISI.— FouKUCcesalvt aUR*Tiiprilovlip|.iiii 'i' .r im. ■ m. iim! jii-uital Of the human remsle fisliis {Tuuruvuxi^ 1, clHi.tiK. •;, tluii.s clllDriilli; \ genlMlflnun:: 4, labia majors: S. anui; 0, rwc jgeHl emlncDPC : 7. Uljlami

The reader is again reminded that in the stage when the 1 cloaca is present, the Miilleriaii ducts terminate in the siniia | I un^nitalis (Plate YII.). As previously stated, the stnus u gsnitalia becomes the female urethra, ita terminal portion expanding into the vestlbulum vagins. The openings of the Miillerian ducts fall widiin tliis latter veatihular region of the sinus. the lower purtidu of the two ducts by this time, however, have fused to form the uf«ru3 and the vagina, and the glans penis, wliilc the integuinentary fold that ])artially encircles the latter assumes more distinctive character as the prepuce. This fold gradually advances over the glans and adheres to it, the adhesion j)ersisting until, or shortly after, birth. All of the rudimentary penis, exclusive of the glans and of the genital folds, becomes the corpora cavernosa of the adult organ. The characteristic structure of the corpora cavernosa is forcshadowcKl as early as the third month by the appearance in the ])enis of capillary blood-vessels, which, in the sixth month, undergo marked dilatation.

The groove on the under surface of the penis becomes dee{>er, and the genital folds, which bound the groove laterally, increase in size. This groove extends from the orifice of the urogenital sinus to the glans penis. The genital folds, which in the female, remain distinct and l>ecome the nymphie, unite with each other in the male and convert the groove into a canal, which latter is practically an extension of the urogenital sinus along the entire length of the j)enis to the glans. The e^uial thus formcnl is the anterior part of the male urethra, or, in other words, it includes all of the urethra except its prostatic i)ortion, whicli represents the urogenital sinus. The orifice of this newlv-formtHl canal, situated in the glans, is the meatus urinarius. Failure of union of the genital folds, either wholly or in part, results in total or in partial deficiency of the floor of the urethra, tliis anomaly being known as hypospadias. If the defective closure involves only the glans, the condition is denominated glandular hypospadias.

The genital folds form not only the si<les and the floor of the penile urethra, but by an extension of their growth, also its roof, thus completely surrounding it. Upcm the acquisition, by the now united genital folds, of bhwKl-vessels and cavernous spa(!es, they become the corpus spongiosum of the j)enis, and thus is established the p<?nnanent or adult relation of these partes.

The genital ridge be(^omes differentiated into two prominent folds or pouches placed one on either side of the root of the penis. In the fourth month these unite to form the scrotum, the line of union being indicated by the raphe. Pailure of union of the two halves of the scrotum is one of the features of certain forms of so-called hermapliroditism.

The glands of Cowper, which correspond to the glands of Bartholin of the female, are developed, like the latter, as evaginations of the terminal part of the urogenital sinus.

The accompanying tabulation exhibits a comparison of the organs of the two sexes on the basis of their common origin. Male and female parts that develop from the same fetal structure are said to be homologous with each other.

Homologies op the Sexual System.

Fetal Structure. Female Organs. Male Organs.

Indifferent sexual gland. Ovary. Testis.

Wolffian body — Its middle series of tu- Short tubules of par- Vasaefierentia^rete testis

bules and ovarium and and coni yasculosi.

Corresponding part of Horizontal or long tube Tube of epididymis.

Wolffian duct. of parovarium.

Keinainder of Wolffian Usually altogether dis- Yas deferens, seminal

duct. appears ; if persistent, vesicle, and ejacula Gartner's duct tory duct

Upper serin of short Stalked hydatid of Mor- Stalked hydatid of Mor tubules (pronephros). * gagni. g^^i Lovoer series of tubules. Paroophoron. Paradidymis (org^an of

Girald^).

Duct of Miiller — Its upper extremity. Fimbria of oviduct. Sessile hydatid.

Succeeding portion. Oviduct Usually disappears ; if

persistent, duct of Bathke. Remaining portion, by Uterus and vagina. Uterus masculinua.

fusion with its fellow.

External Organs.

Fetal Structure. Female Organs. Male Organs.

Genital eminence. Clitoris. Penis.

Genital folds. Nymphae and bulbi ves- Corpus spongiosum, en tibiili. closing spongy part of

urethra. Genital ridge. Labia majora. Scrotum.

Urogenital sinus. Urethra and vestibule. Prostatic urethra, membranous urethra, prostate, Cowper's glands.

Glands of Bartholin.

SUMMARY.

1 . Th(» male and the female internal generatiye organs, as well as the kidney and the ureter, originate from the mesothelial Hning of the b(Kly-cavity, being directly produced from the Wolffian IxhIv and the duct of Miiller.

»

2. The bladder and the female urethra, but in the male only th(» prostatic urethra, result from the metamorphosis of the intrn-ombryonic jmrt of the allantois, and are therefore to be repirded as of entodermic oripn.

3. Ikfore the estiiblishment of the j)ermanent kidney, a temponirily functionating organ, the mesonephros, performs the oflice of a kidney during a jxirt of fetal life, and this latter is preceded i)y the pronephros, an organ which, though represented in the higher vertebrates by a vestigial remnant^ is functionally active only in larval Amphibia and in bony fislu's.

4 The Pronephros. — The mesothelial cells of the outer or pari(>tal wall of the body-cavity become invaginated in a line parallel with the axis of the body, and the cord of cells thus Ibrined bectnnes hollowed out to constitute the pronephric or s('j;;iiiciitnl du<'t. At several points this duct retains its connection with the surface-cells by nieiins of cell-cords, which latliT beeonie tubes and acquire glomeruli. The long tube anid the shorter tubules with their glomeruli constitute the pronephros.

5. The Mesonephros. — The transverse segmentation of the middle plate, whi<'h connects the ]>araxial mesoderm Avith tlit^ parietal plate, results in the formation o( a series of ccll-mass<'s, llu' nephrotomes. Each nephrotome becomes a tube and a<M|uircs one or more glomeruli. The decjwr eniU of the tubes bcc<)me c<»imected with the pronephric

• ir M«n;iiicntal dnct, which latter is known henceforth as

the nicsonephric or Wolflian duct. These tubes, with the atljacent part of the Wolffian duct, constitute the mesonephros, which fnnctii»nat<'s, for a time, even in the human fetus, as an organ of urinary ex<»retion. The entire AVolffian duct, with the pronephric tubules and th(» mesonephric tubules, eonstitntes the Woltlian body. The AVolffian duct ojK»ns at its lower or caudal extremitv into the cloaca.

6. The metanephros or pennanent kidney develops in part from a small diverticulum that pouches out from the lower or caudal end of the Wolffian duct. The straight collecting tubules and the pelvis of the kidnev correspond to the dilated and subdivided fundus of this diverticulum, while the ureter represents its stalk. The secretory tubules of the kidney develop from the inner zone of the metanephrogenic tissue. The surrounding mesodermic tissue furnishes all the component elements of the ureter-walls and of the kidney except the epithelial parts, as noted above.

7. The supraxenal bodies probably are derived, in part, from epithelial outgrowths which proceed from the mesonephros to form the cortical part of the organ, and, in part, from chains of small cells that bud forth from the embryonic sympathetic ganglia to form its medulla. The surrounding mesodermic tissue contributes the connective-tissue parts of the suprarenal body.

8. The sexual apparatus in its earlier stages presents no distinctions of sex. The elements of this earlv indifferent type are the indifferent genital gland, the Wolffian duct, and the duct of Miiller.

9. The indifferent genital gland originates in the mesothelium of the body-cavity. The mesothelial cells overlying the ventromesial aspect of the Wolffian body undergo multiplication in the fifth week and thereby produce an elongated elevation, the genital ridge. Further multiplication of its cells and the addition of other elements bring about the transformation of this ridge into the well-defined genital gland, which now lies in close relation with the Wolffian tubules. The mesothelial cells are the "germinal epithelium of AValdeyer, the cells that produce the ova or the spermatozoa, according to the future sex.

10. The duct of Miiller makes its appearance soon after the AVolffian duct. It lies parallel with and to the outer side of the Wolffian duct and also terminates in the cloaca. It is of mesodermic origin, being produced either by evagination of the mesothelial cells of the body-cavity, or by a splitting off from the Wolffian duct.

11. The generative systems of both sexes result from the metamorphosis of the three structures making up the early indifferent sexual apparatus — namely, the indifiei'ent sexual gland, the Wolffian body, and the duct of MuUer.

12. The male sexual system is ])roduced by the transformation of the indifferent gland into the testicle, and the conversion of the Wolffian tubules and the Wolffian duct into the system of excretory ducts for that gland, the short tubules becoming the va.sa effcrc^ntia and coni vasculosi, while the Wolffian duct itself furnishes the body and the globus minor of the epididymis, the vas deferens, the vesicula seminalis, and the ejaculatory duct. The duct of Muller remains undeveloped and is represented in the adult by the atrophic sessile hydatid and the uterus masculiuus.

13. The female sexual apparatus is brought about by the development of the indifferent gland into the ovary, and by the metamorphosis of the upper segments of the ducts of Muller into the Fallopian tubes, and the fusion of the remaining portions of the two ducts to form the uterus and the vagina. The Wolffian duct and tubules give rise to atrophic structures in the female, the most conspicuous of which is the parovarium or ej)oophoron.

14. Both the male and the female external genitalia are developed from fetal structures conmion to the two sexes, the genital eminence, the genital ridge, and the genital folds. The genital eminence is situated at the anterior or ventral part of the eloaeal depression. The genital ridge is an elevation surrounding this pit and the genital eminence, while the genital folds are on the under surface of the genital eminence, on(» on each side of a longitudinal groove.

15. The Wolffian ducts and the ducts of Muller open into the cloaca, but when that a|>erture becomes differentiated into the anus and the urogenital sinus, as it does at the fourteenth week, these ducts fall to the latter apartment. The orifice of the urogenital sinus being at the base of the genital eminence, the sinus comes into continuity with th(» groove on the under surface of the eminence.

16. The female external genitalia are produced by the further development of the three structures mentioned above.

The genital eminence becomes the clitoris. The genital folds on the under surface of the clitoris become somewhat prolonged to constitute the labia minora. The genital ridge becomes, anteriorly, the mons veneris and laterally the labia majora. The orifice of the urogenital sinus is represented by the vestibule, and since the Miillerian ducts near their termination in the urogenital sinus fuse to form the vagina, the latter passage opens in the adult into the vestibule. Since, also, the urogenital sinus receives the termination of the allantois, which becomes the female urethra, the latter canal likewise opens into the adult vestibule.

17. The male external genitals represent a further development of the embryonic genital eminence, genital folds, and genital ridge than do the female organs. The genital eminence becomes the penis, the genital folds, uniting with each other so as to surround the groove, producing the corpus spongiosum. The groove itself, being thus converted into a canal which extends the now closed urogenital sinus to the end of the penis, constitutes all of the male urethra except the first or prostatic portion. The prostatic urethra represents the proximal extremity of the allantois. Since the Wolffian ducts open into the urogenital sinus after the division of the cloaca, the terminations of those ducts, represented now by the ejaculatory ducts, open into the prostatic urethra ; and since the Miillerian ducts also open into the urogenital sinus, the uterus masculinus, which is the representative in the male of the terminal parts of the Miillerian ducts, is found likewise in the prostatic urethra. The lateral parts of the genital ridge, which, in the female, become the labia majora, fuse with each other in the male to form the scrotum.

18. The condition of so-called hermaphroditism may be produced either by an unusual degree of development of the female external genitals, resulting in a clitoris resembling a penis and in labia majora which simulate a cleft scrotum ; or by the arrested development of male organs, whereby the genital folds and the genital ridges fail to unite, the urethra in consequence opening at the base of the penis.

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CHAPTER XIV.

THE DEVELOPMENT OF THE SKIN AND ITS APPENDAGES.

Thk a]))iGiidatroH of the Kkin include the BebaceooB and sweat glands, tlio manunoi; glanda, the nails, and the halis.

THE SKIN.

isiiitiiig of tlic epiderniU or cuticle and of the lorm, or coritim, is derived from two sources, the epitEiclial epidermis being a prtKhict of the ectoderm, and the corinm originating from themesoderm. The nuils and hairs are outgrowths of the epithelial layer, wliile tlio various glands Are lierived from infoldings or in vagi nations of the same stratum.

The corinm, the connectivetissue i'"m|Mment n( the skin, is an i>nt}:nhwtli of the cntis plates tif the primitive segments or somites (Fig. l;'.;!). If iirst appeju-s ill (Tilde P)rm in the s^'i'inid niontii lis a layer of spindle-cells liemtfitli the ectoderm. In till' third month, the more siipcrlicial imrt of this layer ac<iiiires more definite and distinctive eharaeter, the nilher loose aggn'gation of cells having diilerentiate<l inti> a tissue wich is a niesh-work of handles of white fibrous connective tissue with some intermingled elastic and muscular fibers ; this constitutes the corium proper. The deeper layer of cells becomes a loose, subcutaneous areolar tissue containing a few scattered fat-cells. About a month later the external surface of the primitive corium loses its smooth character and presents numerous little elevations, the villi, which project into the overlying epidermis. The villi, being highly vascular, play an important i)art in the nutrition of the epidermis and being also freely supplied with nerves they sustain an equally important relation to the sensitiveness of the skin.

From the middle of fetal life onward, the fat-cells in the subcutaneous tissue increase in number to such extent that there is formed a continuous and well-marked subcutaneous layer of fat, the panniculus adiposus.

Certain of the cells of the primitive corium differentiate into unstriated muscular tissue, forming thus the muscles of the liair-follicles, the airectores pilonun, as well as the subcutaneous muscular tissue of the dart(»s of the scrotum and penis, and that of the nipple and of the perineum.

The epidermis, consisting of the suj>erficial horny layer and the deeper mucous layer or stratum Malpighii, is entirely an epithelial structure. Its element* are simply the descendants of the early ectodermic cells specially modified to afford the necessary protection to the more sensitive and delicate corium.

The division into the two strata of the epidermis is indicated a? early as the latter part of the first month, at which time the cells of the ectoderm have become arranged into two single layers, a superficial layer of rather large flattened cells and an underlying stratum of smaller elements. The cells of the outer layer, or epitrichiiun, which probably represents the future stratum corneum, successively undergo degeneration and descpiamation, the places of those lost being supplied by the formation of new ones from the deei)er layer. As time goes on, both layers increase in thickness and the hairs and the glands of the skin are gradually formed. With increased proliferation there is increasingly

270 TEXT-BOOK OF EMBRYOLOGY.

active desquamation of superficial cells, and as the degenerate and cast-off cells become mixed with the products of the sebaceous glands, there is formed a sort of cheesy coating of the skin, the vemix caseosa or smegma embryonmn. This is first easily recognizable in the sixth month, and first covers the entire surface of the body in the eighth month. It serves to protect the epidermis of the fetus from maceration in the amniotic fluid.

The completion of the epidermis, aside from the development of its accessory parts, consists simply in further increase in thickness and in the modification of the superficial cells to produce the characteristic scale-like elements of the corneous layer of the skin, accompanied by the differentiation of the deeper cells into those of the rete mucosum or stratom Malpiglm. The extent to which these modifications are carried varies in different regions of the Ixnly.

THE DEVELOPMENT OF THE APPENDAGES OF THE SKIN.

The Nails. — The nails have their beginning in little clawlike projections, the primitive nails, that appear upon the tips of the still imperfect fingers and toes in the seventh week.* These result from localized proliferation of the cells of the epidermis, Ix'ing entirely epithelial structures. The rudimentary nails project from the tips -of the digits, instead of o<!eupying the dorsal position of the completed structures. The claw-like primitive nail, between the ninth and twelfth "weeks, becomes surrounded l)v a groove, which starves to separate it from the ij:eneral eetodermic surface. Th(»se claw-like rudiments of the human nails are (juite similar to the primitive chiws of many mamY\r.. iw- Longitudinal sec- mals, the ])rinntive nail in ea(^li case

tion throuKh the toe of a <'«'r- •it i i i .1 •!

copitiu<ussoiftir<ieKrni.aur^: inchi(hnir a (lorsa! part, the nailn,>.naiH.iato:^A, plantar horn plate, and a portion which belonjrs

(Sohlenhorn) ; 7IM', nail-wall. * , . ,. .

t<) the ventral surra<*e ol the <ligit, called the plantar horn (Fig. l.'U). The striking difference between the nails of the human a<hilt and the claws and

• Or ninth week, Miiiot. ' A j^eniis of lon^MaikMl African monkeys.

DEVELOPMENT OF APPENDAGES OF THE SKIN. 271

hoofs of many animals is due in great measure to the degree of development to which this ventrally situated plantar horn attains. In the hoofed mammals (Ungulata) and the clawed mammals (Unguiculata), the plantar horn undergoes very great development, whereas in man it retrogrades and leaves no trace except the nail-welt, or the narrow line of thickened epidermis where the distal end of the nailbed merges into the ordinary skin. After the atrophy of the plantar horn, the dorsally situated nail-plate being alone present, the rudimentary nail bears a greater resemblance to the adult condition.

As the nail-plate gradually acquires more distinctive character, the deeper layers of the skin specialize into a structure adapted to its nutrition. This is the nail-bed, a highly vascular and sensitive cushion consisting of the corium and of the stratum Malpighii of the epidermis. It is especially from the proximal part of the nail-bed, representing the matrix of the fully-formed condition, that the nail grows. The rate of growth is such that- the ends of the nails protrude beyond the tips of the digits in the eighth month.

The tissue of the folly-formed nail corresponds to the stratum lucidum of the typical epidermis, developed to an unusual degree. The epitrichium or future stratum comeum, the most superficial layer of the epidermis, does not form a part of the nail, but constitutes a thin covering, the eponychium ; this is lost in the seventh month, with the exception of a small band over the root of the nail, which persists for a short time as the perionyx.

The nails of the toes are always somewhat behind those of the fingers in development.

To repeat, the claw-like rudimentarj- nails appear in the seventh week, the nails are perfectly formed about the twelfth week, and break through their epidermal covering in the seventh month, reaching to or beyond the finger-tips in the eighth month.

The Hair. — Each hair consists of the projecting shaft and the embedded root, with its expanded deep extremity, the hair-bulb, the root being embraced by the hair-foUicle. The

272 TEXT-BOOK OF ICMBriYOLOGY.

hair is entirely of ectodermic origlu, being ileriveii from the epidermal Isij-lt of fhi.' Hkin, while the liair-follicle is partly derived from the epidermis and in jjart is a product of the coriiim. The hairs are homologous with the feathers and scales of the lower animals.

The development of the hair is initiated in the third fetal month hy the appearance of small solid masses of epithelium in the stratum Mulpighii of the epidermis. The epithelial plugs or hair-germs grow into the underlying corium and are met by outgrowths or papiUn of the latter, which develop almost simultanoonsly. The papilla; are very vascular and serve for the nutrition of the developing hair.

The root and the shaft of the rudimentary hair result from the specialization of the axial or central cells of the hairgerm. These eells lengthen iu the direction of the long axis of the hair-germ and become hard and corneous, thus constituting the ro'it and the shaft, the cells of the deepest part of the hair-eenn forming the bulb. Tin- irrowtli of the hair

b«Jr; te, liuUiof hBtr; Aa, young hair; of Ihelialt: W, balr-n>nide: M.wbBG<

Lir (Hertwlg): ^ and B, Ir-pnpms : U, ireriD of

in length is due to the proliferation and specialization of the cells of the bulb. The papilla of the niiderlying corinm indents the deep surface of the hair-bulb, this close relation of the two structures enabling the {>apilla the better to fulfil its function of providing nourishment t« the bulb.

THE SEBACEOUS AND SWEAT-GLANDS. 273

The hair-follicle, consisting of an outer connective-tissue portion or fibrous layer and an inner epithelial part, the inner and outer root-sheaths, is partly of mesoderniic and partly of ectodermic origin. The inner and outer rootsheaths are produced by the peripheral cells of the hairgerra augmented by cells contributed directly by the stratum Malpighii of the epidermis. The outer fibrous constituent of the follicle results from the mesoderniic cells of the corium that immediately surround the hair-germ.

Gradually increasing in length by the addition of new cells from the hair-bulb, the primitive hair at length protrudes from the follicle as free hair. This first growth of hair is unpigmented and is extremely fine and soft, being known as the lanugo or embryonal down. This appears upon the scalp and some other parts of the body in the fourth month, gradually extending over the entire surface in the succeeding months. In the eighth month the lanugo begins to disappear, but is not lost as a whole until after birth, when the permanent growth of hair takes its i)lace. Upon the face, in fact, the lanugo j)ersists throughout life.

The development of the secondary hair is still a disputed point. It is claimed by some authorities (Stieda, Feiertag) that they develop from entirely new hair-germs. Most authorities hold, however, that the secondary hair develops from the same papilla that produced the hair just lost. According to this view, the empty root-sheath of the cast-oflP hair closes so as to form a cell-cord which represents a hair-germ for the new hair. The cells of this germ in most intimate relation with the underlying papilla produce the new hair in the same manner that hair is produced by the original hair-germs. As the new hair grows toward the surface the old one is gradually crowded out.

The Sebaceus and Sweat-glands. — The sweat-glands, including not only the sweat-glands proper but the ceruminous glands of the external auditory meatus and the glands of Moll of the eyelids, are derived from the ectodermic epithelium. The glands are of the simple tubular type. Each gland develops from a small accumulation of epidermal cells that

18

274 TEXT-BOOK OF EMBRYOLOGY.

grows, in the fifth month, from the Malpighian or mucous layer of the epidermis into the underlying corium. The solid epithelial plugs l)ecome tubes in the seventh month by the degeneration and final disappearance of the central cells. The deeper part of the tube becomes coiled and its lining epithelium takes on the (characteristics of secreting cells. Some of the cells of the original epithelial plug undergo specialization into muscular tissue, thus producing the inyolnntary muscles of the sweat-glands.

The sebaceous glands arc dcvelopeci from solid epithelial processes that originate from the deep layer or retc inucosum of the epidermis in a manner similar to that of the development of the sweat-glands. There is the difference, however, that the ej)ithelial plugs acquire lateral branches and thus usually produce glands of the compound saccular or acinous variety. There is the further difference that the epithelial outgrowths generally develop from the ectodcrmic cells of the outer sheath of the root of the hair near the orifice of the follicle (Fig. 136, id), in consecpience of which the ducts of the finished glands usually open into the hair-follicles. In some regions, however — regions devoid of hair, as the prepuce and the glans penis, the labia minora, and the lips — the growth is directly from the stratiini Malpighii, as in the case

of the sweat-glands.

The Mammary Gland. — The mammary pflnnd represents a number of highly specialized p:lan<ls of the skin, so associated as to constitute the sinjrie adult structure. Its origin, therefore, is to be sought in the cells of the epidermis in common with that of the ordinary glaiuls of the skin.

It is claimed by many authorities, by (ie^enbauer especially, that the mamnue are modified sebaceous glands ; others assert that they are to be elass<'d with the sweat-glands, Ilaidenhain having shown that in the development of the milk-glands there is no fatty metamor])hosis of the central cells as in the sebaceous glands, and Minot enijihasiziiig tin* fact that their mcwle of development closely resembles that of the sweat-glan<ls.

The development of the milk-glands is begun as early as

THE MAMMARY GLASD.

275

\

the second month. At this time the deep layer of the epidermis, in the bites of the future gknds, becomes thickened by the multiplication of its cells, the thickened patch encroaching upon the underlying coriuni (Fig. 136, A, b). Thia thickened area enlarges somewhat peripherally and its mai^ins become elevated, owing to wliich latter circumstance the piitch appears relatively depressed [B). The depression is known as the Klaadular area., and it corresponds with the fatnre areola and nipple. In many mammals the development of the milk-glands is initiated by the appearance of a

Fio. IM,— Secllunii representing (hree luceeislvc 8t»Be« of dBTclopmont of tha human DiiimmB (ToiirDeui|; .1, fetiaof SlWmm. (l.S la): fi. of 10.16 eni. (4lii,): C, of '^4.3% GtD. (9.A In.); u. epidermis; A. kggregnlloii of epldcmtKl veils funnlDg ■nUgo of gtand : e, gBlnctophoroiu dui^ts; d, groove limiting glamliitsr area; i, (treat pectoral muscle; /, unslrlalcd muBLnilar tisaue of arvolu; g, subCDtlDeoiu adipose tisaue.

pair of linear thickening;s of the epidermis on the ventrolateral aspect of the bfjdy, called the milk -ridges or milklines, from localized thickenings of which the multiple mammary glands of such animals develop. These railklines have also been observed in the human embryo, but the constancy of their occurrence in man has not as yet been established.

From the bottom of the glandular area, numerous small masses or bad-like processes of cells ^row down into the corinm. Some of the buds acquire lateral branches. By the hollowing out of these cell-buds the latter are transformed

276 TEXT-BOOK OF EMBRYOLOGY.

into tubes (c), which open upon the glandular area. The branching of the cords begins in the seventh month and is carried on to such a degree that each original cell-cord gives rise to a tubo-racemose gland. The hollowing out of the solid processes begins shortly before birth, but is not completed until after that event. Each cell-cord becomes, in the strict sense, a complete gland, each such individual structure forming a lobe of the mature organ.

This stage of the human mammary gland — that is, a depressed gland-area upon which open individual glands, the nipple being absent^ — is the permanent condition in some of the lowest mammals, as in the echidna, one of the monotremes. In all higher mammals, however, further metamorphoses occur in the tis.sites of the glandular area, and in the human fetus these tissues become the nipple and the surrounding areola.

The nipple is partly formed before birth, but does not become protuberant until post-fetal life. The depressed glandular area rises to the level of the surrounding parts, and its central region, which includes the orifices of the already formed or just forming ducts, swells out into a little prominence, the nipple. This prominence is a protrusion of the epidermis and includes the terminal extremities of the milk-ducts as well as the blood-vessels and connective- tissue elements which surround tli(» ducts. In the dermal constituent of the rudimentary nipple unstriated muscular tissue develops. The region of the glandular area not concerned in the formation of the nipple becomes the areola.

At birth, as above intimated, the manmiary gland is still rudimentary, since many of the ducts have not yet acquired their lumina nor their full degree of c(mij)lexity. Shortly after birth a small quantity of milky secretion, the so-called witches' milk, may be ex])ressed from the glands — in the male and female infant alike. This is true milk according to Rein and Barfruth, but according to Kolliker, it is merely a milky fluid continuing the debris of the degenerated central cells of those rudimentarv ducts that were still solid at birth.

So far, the milk-glands are alike in the two sexes, but

THE MAMMARY GLAND. 277

while in the male they remain rudimentary structures, they continue to increase both in size and in complexity in the female. The increase aflfeets not only the glandular tissue proper but the connective-tissue stroma as well. At the time of puberty the growth of the glands receives a new impetus, which is very materially augmented upon the occurrence of pregnancy. There may be said, therefore, to be several distinct phases in the development of the milk-glands, first, the embryonic stage ; second, the infantile stage ; third, the stage of maturity beginning at the time of puberty ; and finally, the stage of fhU functional maturity consequent upon preg-> nancy and parturition.

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CHAPTER XV. THE DEVELOPMENT OF THE NERVOUS SYSTEM.

The nervous system of the adult, including the cerebrospinal axis and nerves, and the sympathetic system of ganglia and nerves, is made up of the essential neural elements, the neurons, together with the supporting framework or stroma.^

The neurons and a part of the stroma result from the specialization of the ectodermic layer of the embryo. The ecto<lermic origin of the nervous system acquires certain interest in view of the conditions that obtain in some of the lowest and simplest organisms. For example, in the ameba, the single protoplasmic cell which constitutes the entire individual possesses the several fundamental vital properties of protoplasm, such as resi)iration, metabolism, contractility, motility, etc, in ecpial degree, no single property being more highly developed than the others, and no particular part of the cell exliibiting greater s])ecialization than the other parts. In other words, the j)n)t()plasmic substance of the animal is at once a respiratory mechanism, a nervous apparatus, and an organ for the execution of the various other vital functions.

In somewhat more highly developed creatures, as the infusoria, although there is no differentiation into separate tissues and probably not even into separate cells, there is seen some progress toward the sj)ecialization of certain parts of the orgimism for the performance respectively (»f the different functions of life. For example, the central part of the ani ' The neuronti are the units of which tlic nervous Bvstem is made up. Each neuron consists of a nerve-cell with everything belonging to it -that is, with its various processes, including the nxis-rylinder process or iieuritf which becomes the axis-cvlindcr of a nerve-ri))er. 278

THE DEVELOPMENT OF THE NERVOUS SYSTEM. 279

mal has digestive functions, while it is by the superficial portion alone that the creature is brought into relation with the outside world, the sensitiveness or irritability of the surface, by which the animal is made responsive to external impressions, being the nearest approach to the function of a nervous system that it possesses.

This primitive function of the surface of the organism is suggestive as to the origin of the nervous system of higher type creatures. It will be seen, indeed, that not only is the nervous system proper derived from the ectoderm ic cells of the embryo but that the peripheral parts of the organs of special sense, as the olfactory epithelium, the organ of Oorti, and the retina, have the same origin.

The alteration of those cells of the ectodermic stratum that are to specialize into nervous elements begins prior to the fourteenth day in the human embryo, in the stage of the blastodermic vesicle. The change consists in a gradual modification of the form of the cells, the cells common to the general surface of the germ assuming the columnar type. The process affects the cells of the median line of the embryonic area in advance of the primitive streak, resulting in the production of a thickened longitudinal median zone. This thickened area is the medullary plate (Fig. 41, p. 70). On each side of the plate — which is apparent at the fourteenth day — the adjoining ectodermic cells become heaped up to form the medullary folds, which latter therefore bound the medullary plate laterally. The medullary plate becomes concave on the surface, forming the medullary groove (Fig. 137). By the deepening of the groove, the lateral edges of the plate approach each other (Fig. 138), and finally they meet and unite, thus producing a tube, the neural tube or canal.

Since the medullary folds similarly meet and unite with each other — their union slightly preceding that of the edges of the plate — the neural tube comes to lie entirely beneath the surface-ectoderm and soon loses all connection with it. The closing of the tube and the union of the medullary folds occur first near the anterior end of the embryonic area, in a position that corresponds with the region of the future neck,

2S*I TEXT-ROOK fiF EMUnYOLOGY.

flnil fnim this jwint it proceeds Iwlh cephalad and caudad. Sinw the nie»!ullar>' fnUis at their caiidal extremity embrace

ITctKkarJ, Scmitr. Cnl /•laJrrm.

Fid. is;.— TrBDirene lecllon of > ■Ixteen-Rnd'a-l

mbryo poaMctlOK

the primitive streak {Y\g. -11, p. 70), the latter structure i» inchidefl within the cjuidiil end of the neural tube by the \

isHTie BBcUiiii of II fitl<;i;n-mnl a-hslf-dnj sheap embryo Kvca lomltci (Bonnet).

coming together of the folds, and thus the blastopore, which was previously the external aperture of the archonteron.

THE DEVELOPMENT OF THE SPINAL CORD. 281

comes to constitute the neurenteric canal, or an avenue of communication between the neural canal and the primitive intestine.

The neural canal then is a tube composed of columnar cells, which is formed by the folding in of the ectoderm and which occupies the median longitudinal axis of the embryonic area and consequently of the future embryonic body. From this simple epithelial canal the entire adult nervous system is evolved.

The evolution of the highly complex cerebrospinal axis from such a simple structure as the neural canal is referable both to the principle of unequal growth — the walls of the tube becoming thickened by the multiplication of the cells — and to the formation of folds.

The portion of the neural canal — approximately one-half — that is devoted to the formation of the brain is delimited from the part that produces the spinal cord by the dilatation of the anterior or head-end of the tube, and the subsequent division of this dilated sac-like portion into three communicating sacs called respectively the fore-brain, mid-brain, and hind-brain vesicles (Fig. 142). These three vesicles give rise to the brain, while the remaining part of the neural canal forms the spinal cord.

THE DEVELOPMENT OF THE SPINAL CORD.

In the growth of the spinal cord from the spinal portion of the neural canal we have to consider the evolution of a cylindrical mass of nerve-cells and nerve-fibers with the supporting stroma from a simple epithelial tube.

The wall of the neural tube, although consisting at first of a single layer of epithelial cells, is not of uniform thickness throughout its circumference. While the external outline is oval, the lumen of the tube is a narrow dorsoventral fissure (Fig. 45, p. 73). The cavity is therefore bounded on the sides by thickened lateral columns, while the dorsal and ventral walls, which connect the lateral columns with each other, are thinner and are called respectively the roof-plate and the floor-plate.

282 TEXT-BUOK UF EMBRYOLOGY.

After a short time, the walls of the tube having thickened by the multiplication of the cells, the shape of the lumen alters, two laterally projecting angles being addetl (Fig. 139). The effect of tiiis change is to partiallydivide each lateral half into a dorsal and a ventral region. The neural canal at tlii,-i stage may be said to consist of six columnri of cells, the two dorsal zones connected with each other liy the roof-plate, and the two ventral zones united by the floor-plate. These regions are also distinguishable, with . certain characteristic modifi<a- I tions, in the head-region of tlw I tnl>e. They are important in their bearing upon the further development of the atrnctnre, since the dorsal and ventral zones are related respectively to the dorsal or sensory and the ventral or motor roots of the spinal nerves.

The differentiation of the cells of the neural tube into two ^ kinds of elements, one of which gives rise to susteutative tissue or neuroglia wliile the other produces the nerve-cells, is ( observed at about the end of the third week. The single , layer of columnar cells which at first comjMises the wall of the tube, the long axes of the cells being radially arranged, soon exhibits near the lumen a row of roond cells, jirobably the first offspring of the columnar cells. The round cella are the genn-cells or Kerminating cells, from which are develoiied ihe neuroblasts or young nerve-colls as well as the neuroglia cells. All the other cells, known as the spongioblfista or ependymal cells, are concerned in producing susten

36<rWim Kaillkcr): c.ccDtraJ t, its eplthdUI nntng: Me' ortyl, Iho original placu of ot (he cuul ; a, the wblte lU of the anterior coluouu : a. g atance of BnlerolUiiral born terlar column ; or. miterlui pr, iMatarior roola.

tati\

ssue.

Tiie stroma of the central nervous system includes two constituents — a connective-tiBsue element, and a part, the neuroglia, which is of epithelial origin, and which is not to

THE DEVELOPMENT OF THE SPINAL CORD. 283

be regarded, therefore, as connective tissue. The connectivetissue portion of the stroma is produced by the ingrowth of the pial processes from the pia mater, and is hence of mesodermic origin.

The neuroglia is derived from the spongiobhtsts, which result from the specialization of the large colnmnar cells of which the wall of the neural canal is composed. These cells, whose length comprises the entire thickness of the wall of the tube in the earliest stages, undergo partial absorption and disintegration, each cell being transformed into an elongated system of slender processes or trabecule, and each such system

no. IW.-CrOBB-Bectlon through the ipinal cord of ■ Tertebnite embryo {after His}: a. outer nmltlng membrane; b, outer DeurogUa layer. re«lon of future white matter; e, germ^cells ; d, central canal: e. Inner UmitlnE membrane or ependymal t>rer 1 /, spongioblaiU : a, neuroblaata (mantle tajer) : A, anterior root-flben.

being a completed spongieblaet or ependymal cell (Fig. 140). Thp inner ends of the spongioblasts coalesce with each other, forming thus the Inteinal limiting membrane, while the peri])heral extremities interlace with each other to form a close network, the marginal velum. As the walls of the neural tube increase in thickness, the spongioblasts become mors and more broken up to form the delicate neurogliar networf

284 TEXT-BOOK OF EMBRYOLOGY.

with interspersed nucleated glia cells, which latter are derived from some of the round cells noted above as lying near the hmien of the neural tube and which have taken a position in the marginal velum. Such of the spongioblasts as border the cavity of the neuml tube become the cells of the later ependyma of the central canal of the spinal cord and of the ventricles of the brain. The cells of the ejwndyma become ciliated in the human fetus in the fifth week.

The nerve-cells of the spinal cord — as also of the brain — are the specialized descendants of the germ-cells referred to above. The proliferation of the germ-cells produces the neuroblasts, or young nerve-cells (Fig. 140). The latter elements move away from the primitive position of the germcells near the lumen of the tube and, taking up a position between the bodies of the ei)endymal cells and the periphery of the neural tube, develop into the nerve-cells. The transition is effected by the accumulation of the cell's protoplasm on the distal side of the nucleus and its elongation into a process. This ])rocess is a neurit or axon or axis-cylinder process and is the beginning of a nerve-fiber. The dendrites or protoplasmic processes apj>ear considerably later. Some of the fibers thus prcHluced grow out from the neural tube to constitute the eiferent filxTs of the jxTiphcral nerves, that is, the ventral roots of the spinal nerves, while others contribute to the formation of the fiber-tracts of the cord.

After the appearance of the neuroblasts and developing nerve-cells, the wall (►f the neural tube is divisible into three layers (Fig. 140): an inner or ependsrmal layer, next the lumen of the tube; adjoining this, the mantle layer, made up of neuroblasts ; and a peripherally situated neuroglia layer or marginal velum, which occupies the position of the future tracts of white fibers (►f the cord.

The alterations in the form and size of the sj)inal cord go hand in hand with the histological changes noted above. While th(»se areas that have been mentioned a> the dorsal and ventral zones in{.Teas<» greatly in thi<'kness, the floor-plate and the roof-plate — the ventral and dorsal walls of the neural tube — remain thin i Fig. 141). They are n<'ver invaded by the nerve-cells, but consist of thin layers of neuroglia which

TUE DEVELOPMENT OF THE SPINAL CoRD.

285

later become penetrated by nerve-fibt-ra thatpro"' fnim oiif side to the other. They thus represent the anterior and posterior white commiBsmes of the cord. Thetie plates remain relatively fixed in position because of their failure to expand, while the liitcral walls iif the tulie undergo great expansion, in both the ventral and dorsal directions, as well as laterally. In this way a median longitudinal cleft is produced on the ventral wall of the spinal c<>r<l and a similar one on the dorsal wall. These are the anterior and posterior median fissures. Since the so-called jKisterlor median tissiiit is not a true fissure but merely a eeptiim, it differs from the anterior fissure, and It is held by some authorities that this septum is

formed by the gmwing togetiier of the walla of the dorsal part of the central canal.

The flber-tracts or white matter of the spinal cord develop in the outer or neuroglia layer, each filjer being the elongated neurit of a nerve-cell. Some of the fibers originate from the nerve-cells of the cord while others grow into the c«rd from other sources. As examples of the former method may be cited the direct cerebellar tract, composed of the axons of the cells of the vesicular column of Clark, and the tract of ,*

286

TEXT-BOOK OF KMBRYOLOVY.

GrowtT, made up of the axons oi' cells t>f the dorsal gray horn ; while the direct und crosat-d |iyniniiilul tracts ai-e the axons of cells in the cortex of the ct-rebrum. and the tracta of Goll and of Biinlach are composed largely of the axons >] of the cells of the Hpinal ganglia (see p. 318), The devcl- 1 opmont of these filwr-tracts is not complete until the tilwrs ap(|iiirc their niyeltn-sheaths (see p. 414), The myelination of the tracts of Biirdach and of Goll occurs In the latter part of the fourth month and in the tit\h month ; of the : direct cerehellar tract, in the seventh month ; of ihe jiyramidal tracts, at or soon after birth.

As the walls of the neural canal thicken through the mul-^ tipliciition of the cells, the cavity of the tube is gradually] encroached upon almost to obliteration. Whea development f is complete, all that remains of the cavity is the sm&U ceatral f canal of the spinal cord.

The lengtb of the spinal cord in the fourth fetal month \ corresponds with that of the spinal column, Its lower termi- J nation being opposite the last coccygeal vertebra. From this 1 time forward, however, the cord grows less rapidly than does J the spinal column, so that at birth, the cord terminates at the4 last lumbar vertebra, and in adult life at the second lumbar 1 vertebra. This gradually acquired disproportion in thai length of the two structures explains the more oblique 1 direction of the lower spinal nerves as comi«ired with those g

E higher up. In the early condition of the cord, each pair of ' Uerves passes almost horizontally outward to the cprrespoading intervertebral foramina, but us the spinal column gradn- ] ally outstrips the cord in growth, the lower nerves necessarily ^ pursue a successively more oblique course to reach their j foramina, the lower nerves being almost vertical in direction and constituting, collectively, the canda equina. -J (lev dih ves nei

THE DEVELOPMENT OF THE BRAIN.

The encephalic portion of the neuml iuIk, — that part devoted to the production of the bniin — after undci^ing dilatation, becomes marked "ff into ihe thnc ])riman' brainvesicle«, the fore-brain or prosencephalon, the mid-brain or mesencephalon, and llie hind-brain or rhombencephalon, by constrictions in the lateral walls of the tube (Fig. 142). the constricted part of the hind-brain that adjoins the midbrain is the isthmns. This division occurs at an early stage, before the closure of the tube is everywhere complete. The vesicles communicate with each other by rather wide openings. As in the spinal part of the neural canal, the walls of the primary brain- vesicles consist of epithelial cells, and it is by the. muliiplicalion of these cells in unequal degree in different regions^ and by the fo)*r)iation of folds in certain localities, that the various parts of the adult brain are developed from these simple epithelial sacs.

The stage of three vesicles is soon succeeded by the fivevesicle stage, the primary fore-brain vesicle undergoing division into two, the secondary fore-brain (telencephalon) and the inter-brain (thalamencepalon) or diencephalon, and the primary hind-brain vesicle likewise dividing, a little later, into the secondary hind-brain (meteneephalon) and the after-brain (myelencephalon).

The division of the primary fore-brain is preceded by the appearance upon each of its lateral walls of a small bulgedout area which soon assumes the form of a distinct diverticulum. This is the optic vesicle, the earliest indication of the development of the eye (Fig. 142). In the further process of growth the base of attachment of the optic vesicle becomes lengthened out into a relatively slender pedicle, which remains in connection with the lower }>art of the hiteral wall of the brain- vesicle. Following the appearance of the optic vesicle, the anterior wall of the primary fore-brain vesicle projects as a small ovagination, which latter is then distinctly marked off from

Anterwr hrtUn-wtieU.

Middle brain-vesicle. P0Uerior breun-vesicle.

Fare-brain.

Primary optic vesicle.

Simik ^ optic vesicle. Inter-brain. Mid-brain. Hind-brain.

After-brain. Fore-brain.

Primary optic vesicle.

Jnier-brain. Mid-breun.

Hind-breun.

J^er-brain.

Fio. 142.~Diagrram8 illustrating the primary and secondary segmentation of the brain-tube (Bonnet).

the parent vesicle by a groove on either side. This anterior divertieuhim is the secondary fore-brain vesicle or the vesicle of the cerebrum, and the original or ]>riniary fore-brain vesicle is now the vesicle of the inter-brain.

The division of the primary hind-brain is eflTected by the development of a constriction of its lateral wall, this resulting in the production of the secondary hind-brain or the vesicle of the cerebellum, and the after-brain or the vesicle of the medulla oblongata.

While the three primary vesicles at first lie in the same straight line, they begin to alter their relative positions shortly before division. The change of position is coincident with the flexures of the body of the embryo that occur at this time. Three well-marked flexures appear, the result being

InUr-brain.

Fore-brain.

Cephalic flexure.

Mid-brain.

Olfactory lobe

Optic stalk.

I I I

Cerebral portion of Pontine pituitary body. fltxure.

Fig. 143.— Diagram shuwin^ relations t)f l)raiii-vesiclvs ami flexures (Bonnet\

that the fore-brain is bent over ventrad to a marked degree. The most anterior of these flexures, and the first t4) develop, is the so-called cephalic flexure (Fig. 143), the primary forebrain, in the advanced stiite of the curvature, being bent around the termination of the chorda dorsiUis so as to form a right angle, and later, after its division, an acute angle with the floor of the mid-brain. This curvature makes th(» mid-brain very prominent as regards the surface of the embryonic body, producing the parietal elevation or the prominence of mid-brain. In the region of the future pons Varolii, on the floor or ventral wall of the secondary liind-brain, is a second wullmarked an^^iilarity. This is the pontal flexure. Its convexity projects forward.

A third bond, the nuchal flexure, is a less pronounced cnrvature at the jniicture of the after-brain with the spinal part (if the neural tube.

The Metamorphosis of the Fifth Brain-vesicle.— The fifth brain-vesicle, the caudal division of the primary hind-brain, (lifferentiates into the strnctnres whioh surnmnd the lower half of the fourth ventricle, these stnictures con

F\a. 144.— Diagram of > sinrlttal section or the brain of ■ mammal, sbowlng tbe trpc of stroctiirc and tlic pailB that develnji from the Beveral bialn-TeBlclei (modified rrnm Edltim^r).

stituting the mTelenceplialoii (Pig. 144). The Ustological clianges eorresjjond essentially with those that occur in the spinal segment of the neural tube, the aerve-cells and fibers and the neuroglia resulting from the diilbreiitiation of the original ectoderniic epithelium of which the wall of the tube is composed, and the coimective-tiflaue stroma growing into these from the surrounding mesoderm.

There is a marked disproportion between the rate of growth of the tube in different parts of its circumference. The great thickening of the ventral and lateral walls produces the several parts uf the mednlla oblongata. In the dorsal wall growth occurs to such slight extent that the wall in this region remains a thin layer of epithelium. As a consequence, the cavity of the neural tube is not encroached upon on its dorsal side and the central canal of the spinal cord therefore expands in the myelencephalon into a much larger space, the lower half of the future fourth ventricle. This relative expansion of the central canal begins to be apparent in the third week in the human embryo, from which period it continues to increase. A cross-section through the lower part of the developing medulla shows a cavity which is narrow laterally but which has a considerable anteroposterior extent. A section at a higher level disclosc^s a triangular space, the base of the triangle being the dorsal wall of the cavity.

At the time when the cavity of the after-brain acquires a distinctly triangular shape — about the third week — each thickened lateral half of the tube is divisible into a ventral and & dorsal segment, these being known respectively as the basal lamina and the alar lamina (Fig. 145).

The first indication of the longitudinal fiber-tracts of the medulla is presented by two bands of fibers which appear upon

the surface of the alar lamina and which constitute the ascending root of the fifth nerve and the ascending root (funiculus solitarius) of the vagus and glossopharyngeal nerves. These are later covorcil in by the folding over of the dorsal part of the alar lamina (Fiff. 146) and thnaighupiHT part core- tlius coiiic to ()crn)>y tlicir permanent bviiar rtri<m. of tho po^jticm ill tlic interior of the medulla.

fourlh venlrielo <.f an * /» i i i •

omhryo (His): r, roof of 1 hc parts of the alar lamina^ that are

mv.rai ;a""i : "/. "i"r f^^i^j^.^j ^,^,^.,. j^^ ^\^^ manner referred

Iniiiina ; W, basal luiiiina ;

r. ventral ixjrdiT. to diiliTeiitiatc for the most part

into the restiform bodies or inferior peduncles. These are distinguishable in the third month. The anterior pyramidal tracts develop from the ventral parts of the basal lamiiue and are recognizjible in the fifth month. CoiiK*i<leiitally with the f\)rmation of the fibers, the gray matter of th(* medulla assumes its prrmancnt form and arrangement. This gray matter, although in part ])eculiar to the

nie<1iilla, is in great measure hut the continuation of the gray matter of the spinal cor*l rearranged and differently related because of the motor and sensory decussations and of the dorsal expansion of the central canal. A notable feature of this

■t/ {Ills) : V, ventml border : (, tenls : ot, otic vesicle ;

rearrangement is the presence of masses of gray matter immediately beneath the floor or ventral wall of the now expanded cavity or fourth ventricle.

As stated above, the dorsal \vall of the aftcr-brain vesicle remains an extremely thin epithelial lamina, and the cavity in consequence expands toward the dorsal surface. Owing to the excessive delicacy of this dorsal wall of the cavity, it ia easily destroyed in dissection, with the effect of disclosing a triangular fossa (Fig. 151) on the dorsal surface of the medulla, which in connection with a similar depression on the dorsal surface of the pons, constitutes the rbomboidal fossa, or the foortli Tentride of the brain.

It is often stated in descriptions of the medulla and fourth ventricle that the latter is produced by the opening out of the central canal of the cord to the dorsal surface. It should be borne in mind, however, that the central canal does not, in reality, open out to the surface, although it may appear to do so l)ecause of the attenuated condition of its dorsal boundary. The thin epithelial roof or dorsal wall of the aflerbrain l>ecomes adherent to the investing layer of pia mater, thus forming the tela choroidsa inferior, which roofs over the lower half of the fourth ventricle (Fig. 144). The piamater invnj^inUcs ilu» epithelial layer to form the choroid plexuses of the fourth ventricle. Although apparently within the cavity of the ventricle, the choroid plexuses are excluded from it hy the layer of epithelium, the morphological roof of the after-hrain, which they have pushed before them.

AVhile, for the most part, the roof of the after-brain consists of the thin epithelial layer referred to above, there are slight linear thickenings, the ligulsB, along its latei'al margins, and at its lower angle, the obex. At the up|)er margin of the roof, at the place of junction with the hind-brain, there is also a thicken<>d area, the inferior medullary velum. These regions of thick<T tissue serve to eife<'t the transition from the thin epithelial layer that helps to form the inferior choroidal tela to the more massive boundaries of the rhomboidal fossa.

The Hind-brain Vesicle or Metencephalon. — The

metencephalon <*onsists of the pons, the cerebellum with its suj)erior and mid<lle jx^duncles, and the valve (valve of A"ieuss(Mis). the>e structures are j)r()duced by the thickening of the walls of the fourth or hind-brain vesicle.

Th(» pons is forni<Ml by the thickening of the ventnd wall of the vesi(rlc. Its tnmsverse libers become recognizable durintr the fourth mouth.

The cerebellum grows iVom tin? posterior part of the nK)f or dorsid wall of the vesicle (Fig. \A\). The lirst indication of its development is seen as a thick transverse ridge or fold on th(; po>terior extremity of the <lorsid wall (Fig^*. 147, 1 4S). In tli<* tliinl mouth the <M'ntral j)art of this ridge, now grown larger, )>r('S('uts iour deep transverse grooves with the n'sult oi* dividing the originid eminence into five transverse ridges. The grooved ))art of the ridge is the ]>ortion that subse<|ueutly becomes the vermiform process or median lobe of the cerebelluui, while the smooth lateral ])ortious becom<' the lateral hemispheres. As the vermiform process increases in bulk, two of the ridges come to lie ujM)n its upper surface and three? on the inferior aspi»ct. These ridg(»s and furrows j)ersist throughout lifi^ as the principal convolutions and fissures of the vermiform process (Figs. 14J», 1 :»<)).

The lateral parts of the primary ridge inercaso in size and eventually, in the hiiin&n bmin, outstrip the niwlian lobe in pritwlh. They acquire their chief transverse fifisures in the fourth or fifth iiKinth, and the smaller sulci later.

The thickened cerebellar ridge nn the roof of the hindbrain vesicle being continuous with the lateml walls, the continuity of the cerebellar hemispheres with the jHins through the middle and superior cereliellnr |>eiliincles and with the medulla by means of the inferior pcdnnrles. is easily

thickens and dcvelojxs into the cerebellum, all the remaining part of this roof remains relatively thin and becomes the anterior medullary velum or the valve of Vieussens (Fig. 144). The relations of this structure in the mature brain, stretching across, as it does, from one sujHjrior cerebellar })eduncle to the other and l)eing continuous posteriorly with .the white matter of the cerebellum, ixw. easily explained in the light of the fact that all these parts are but the specialized dorsal and lateral walls of the hind-brain vesicle. Since the roof of the hind-brain vesicle is continuous with that of the after-brain or fifth vesicle, it will be seen that the cerebellum must be in continuity with the roof of the medullary part of the fourth ventricle. The transition from the cerebellum to the epithelium of the tela choroidea inferior is eifected by a pair of thin crescent-shaped bands of white nerve-matter which i>ass downward from the central white-matter of the cerebellum, and which are collectively known as the inferior or posterior medullary velum. Thus, as the result of unecpial growth, there are ])ro<luced from the continuous dorsid walls of the fourth and fifth vesicles the thin laminar medullary velum or valve, the massive cerebellar lob(»s, the thin bands known as the infi^rior niedullarv velum, and the single layer of epithelimn which, with a layer of pia mater, constitutes the inferior choroidal tela.

Although the fourth and fifth brain-vesicles are at first delimited from each other by a constriction, this constriction, as development goes on, <Iisappears, the cavity of the fourth vesicle and that of the iifth together constituting the fourth ventricle of the brain.

The walls of th<» fourth or hind-brain vesicle then give rise vent rally to the j)ons, latendly to the superior and middle cerebellar peduncles, and dorsally to the valve* and the cerebellum, while its cavitv beciunes the anterior half of the fourth ventricle.

The Mid-brain Vesicle. — The third brain-vesieh? or the vesicle of the mi<l-l)rain or mesencephalon gives rise to the structures surroun<ling the acpieduct of Sylvius, the ])ersistent part of the cavity constituting the aqueduct itself.

The thickeninir of the ventral wall of the vesicle results in the formation of the crura cerebri and the poaterior peribrated lamina nr space included between them. The crura first become apparent in the third month as a j»air of rounded longitudinal ridges on the ventral siirface of the vesicle. These remain relatively small until the fifth month, when the longitudinal fibers of the pons begin to grow into them. After this occurrence their increase in size is comparatively rapid, their ventral parts or cmsta becoming separated from each other ami iiidndiiig between tiieni the posterior perforated lamina.

The roof or dorsal wall of the mid-brain vesicle undergoes considerable thickening (Fig, 147), especially in the Sauropsida (birds, reptiles, fishes). In the fifth week a longitudinal ridge appears upon the dorsal wall, which in the third month is replaced by a furrow. The expansion of the wall on each side of the furrow produces a pair of rounded eminences (Figs. 148-151), which, in birds, attain to a much

Fig. UK.— Brain aire; f 6, foro-brafu ; lb, brain: P, ruliln uf pLa

greater development than in mammals and constitute the corpora bigemina or optic lobes. In the human emhr}'o, each of these elevations is divided into two by an oblique groove, and thus arc formed the coipora qtiadiigemina, which are peculiar to man and other mammals.

The jiart of the dorsal wall of the vesicle that underlies the corpora quadrigemina is the lamina qnadrigemina.

The thickening which the walls of the vesicle undergo to produce the several parts of the micl-bmin encroaches so miicli iiiK)n its cavity thatan I'.xceedinj^'ly small cjiual, the aqueduct of Sylvius, remains. It is scarcely necessary to piHiit out llijtt llii>; canal is a part of the ventricular system of the iiniii), ost:ihli'-hiu<;a ctminiunicatiou l>etweeu the fourth ventricle ami the thini ventricle or tyivity of the intcr-braiu. The Metamorphosis of the Inter -brain "Vesicle. — The inter-limiii vesicle results fn.m the division of the primary fore-brain vehicle, comprising what in lell of the latter after the outgrowth from it uf the diverticulum that l>ecome8 the secondary fore-brain. The thickening of the walls of the inter-brain vesicle produces the sirueture-s which surround the third ventricle in the mature eoudition, and which constitute collectively the thalomencephalon or inter-brain, the cavity of tJie vesicle persisting as the adult third ventricle. These Btructures are the optic thalajoi, which iirc iorincd from the lateral walls; the velum interpoBitum and the pineal bod7> which develop from the roof; and the lamina cinerea, the

Fig. ]4S,~A. mualsl icrtiDii IhroURh bniiii <i[ a huiunii rclUB of two-Bud-a-bfttf months (Hla): cA. cerebral lii'mlnphi-'ru ; o. ituWc UinliiiDue:/Hi. ri>niin«n of Monro; o{f, olOwtory tobc: p, pllultar; body ; no, minlulU (iblongaU: eq, corpora quadrlpmlDai tb, eerebetlum, B, brain of human Celui of (hrve montbi (HI*): olf, olftntory Inlie; rM, rnrpUB sltUlum; eq, corpora quadriffemlna ; eft, eerebBllnrnj inn, mcdiinn obloiiKOU.

tuber cinereum, the infiindibuluin, the posterior lobe of the pituitary body and the corpora albicantia, which are differentiatoil fiiuii the floor of the vesicle.

The lateral walls of the vesicle undergo the most marked thickening. The cell-multiplication here is so r.ipid that each lateral wall is converted into a large ovoiil inaris of cells with iiiterminglctl bands of fibers, the optic thalamus.

The roof of the inter-brain vesicle, in nutahle coiitra.st with the lateral walls, remains extremely thin throiighnnt the greater part of its extent (Fig. 144). Fmm ihe Ijack part of the roof, at a point immediately in front of the lamina iiiiadrigeiuina of the mid-brain, a diverticulum grows otit and becomes metamorphosed into the pineal tody. With this e.\ception, the roof of the vesicle reinains a single layer of epithelium, just aa in the ease of the roof of the afterbrain. This epithelial layer adheres closely to the pia mater, which covers it in common with the other parts of the hrain. As the fore-brain expands, it covers the inter-brain, the under surface of the cerebral hemispheres of the former l>eing closely applied to the roof of the latter. As a consetjucnce, the pia mater on the under surface of the fore

Fin. i:iO.—itra<n of A^tm of ihreemnnihs, enlarged. Tbc outer wall of the light

hcmlipheru hu been lemoveil ; LH, left bemlHpliere ; Ca, part of corpiu ■Irialum; FS, site of fossa of Kylviiis; I', vascular fold of pia mater which has been InvagInalcd ihniuRh the mesial wall of the hemisphere: Mb, miil-broln; C.ceiebellum; Jf , medulla oblongala,

brain is brought into contact with and adheres to the pia covering the roof of the inter-brain. Thus the thin epithelial roof of the inter-brain becomes closely united with the two layers of the pia luater to form the velnm iutarpodtam or tela choroidea anterior or superior of adult anatomy. Obviously, the edges of the velum interpositum rest upon the optic thalami, and its piamatral layers are continued into the cavities of the lateral ventricles (Fig. 150). The space occupied by the velum is designated the transyerse fissure of the brain, and it is often stated that the pia mater is pushed in from behind, between the optic thalami and the cerebral hemispheres. As will be seen from the above description^ however, its development really begins in front.

The pineal gland or conarium develops from the back part of the roof of the inter-brain at its point of junction with the mid-brain (Fig. 144). This body is found in all vertebrate animals except the amphioxus, but its form varies greatly in difierent groups. In all cases it begins as a small pouch-like evagination from the roof of the inter-brain, the diverticulum being directed forward. In the human brain alone the structure is subsequently directed backward, so that it conies to occupy a position just over the corpora quadrigemina. This peculiarity of location is due probably to the greater development of the human corpus callosum, by whicli the conarium is crowded backward.

In selachians (sharks and dog-fish), the enlarged vesicular end of the diverticulum, which is lined with ciliated columnar cells, lies outside the cranial capsule and is connected with the inter-bruin by the lonij: hollow stalk which perforates the roof of the (M-aiiiuni. In many reptiles, the conarium is more liighly specialized. In the chameleon, for example, the peripli(;ral extremity has the form of a small closed vesicle which lies outside the roof of the cranium and which is covered by a trans|)aroiit pat(*li of skin. The stalk in this case is ])artly a solid cord and ]>artly a hollow canal, which latter oj)ens into the cavity of the inter-brain. The solid portion lies within a foramen in the pjirietal bone, the parietal foramen. A farther modification of the conarium is jiresented in lizards, blind- worms, and some other reptiles. In these the vesicle underji^oes a marked specializjition, its peripheral wall being so nicxlilled as to become trans))arent and to resemble the crystalline lens of the eye, while the opposite deeper wall comes to consist of several layers of cells — some of which become piirmente<l — ainl ac(juire«* a striking resemblance to the retina. The stalk of the body, which perforates the roof of the skull and is attacheil to the roof of the interbrain, bears a certain likeness to the optic nerve, being solid and composed of fibers and elongated cells. The presence of the transparent epidermal plate which covers the vesicle serves to complete the similarity of this particular type of pineal body to the eye of vertebrate animals. It is for this reason that it is often designated the pineal or parietal eye and that it has been looked upon as a third or unpaired organ of vision.

In man and other mammals and in birds the pineal diverticulum does not reach the degree of development that is attained in certain of the Reptilia. The evagination from the roof of the inter-brain begins in the sixth week in the human embryo. The peripheral end of the process enlarges somewhat and small masses of cells project from it into the surrounding mesodermic tissue. These cellular outgrowths, giving off secondary projections, become converted into small closed follicles lined with columnar ciliated cells. The follicles in the case of mammals very soon become solid or nearly so by the accumulation of cells in their interior. Solid concretions of calcareous matter, the so-called brain-sand (acervulus cerebri) are found in the follicles in the adult. By these alterations the pineal body of birds and mammals acquires a structure resembling that of a glandular organ. Since it is onlv the end of the diverticulum that becomes thus altered, the remaining part constitutes the relatively slender stalk of the pineal body, the stalk being solid at maturity except at its point of attachment to the inter-brain, where a portion of the cavity persists as the pineal recess of the third ventricle.

The pineal body of man and the higher vertebrates is therefore a rudimentary structure and is the representative of an organ that is much more highly developed in some of the lower members of the same series. Its true significance is still a matter of conjecture. Although resembling the eye in its structure, and although regarded by some on that account as primitively an organ of vision, it is considered probable by others that in its most highly developed condition it is an organ of heat perception.

The floor of the inter-brain vesicle presents several interesting

iiu'tainorphosos. Tlui anterior ])art of the floor n?mains quite thill an<l Ix'coines the lamina cinerea of th<^ niatinv hraiu (Fig. 111). Iiuiiiediately posterior to this region, the floor of the vesiele poiK^hes out, this evagination developing into a slender IhIm', (he inftmdibuluin. Behind the [XHut of origin of the ini'undilMihiin a sc>eond protnheranee indicrates the beginning nf* (lie tuber cinereum. By subse(|uent altenitions, the tuber eiurreiini enlarmnir in ('ircuniferenee so as to include the point ol'ori<::in of the infundibnlnm, the base of attachment III" the infnn(lil)nluin eonies to be the center of the tuber riniTruni, so that the cavity of the former is a continuation of thr eavilv of the latter. the end of the infundibulum limiMies tiie posterior lobe of tiie pituitary body or hsrpoMliynlH ( Vi\r>, 144 and 140). Posterior to the tulxjr cinertiiiiii a small evagination of the floor of the vesicle .ippiMi'i an<l berimes divide<l in the early part of the fourth iiiniilh iiiln two lateral halves bv a median furrow. The Iwii bllh' bodies thus forme<l become, after further developiiH lit. ihr corpora albicantia.

I hr hypophysis or pituitary body briefly referred to above iii|iiiir: iimrr <'.\tende(l consideration because of its morlihuiti^firid liiipnriancc. The posterior lobe of this body is the • iil.ii^fiil nid of the infiindihulnm, which is an evagination of I hi iImiii 111' I he inlcr-brain. The cells in the lower end of I hi iiitiiiiihbiihnu specialize into nerve-cells, and ncrvelilii I < .il:ii drv<'h»p. In some lower vertebrates these eleiiii 111 < .111* i-i'iiiiiird throu(rhout lii'<\ but in man and the hi;'. hi I l\pi- niiiiiial> the distinctively nervous character of ihi II- 111 • I- -ooii lost, and the cavity of this part of the ihiiiiiilihiihiiii iill'ris oblitenition. The bmnched ])igmentiilh •iiiir(iiiir-i nM'oiTiii/jihh' in the j)osterior K)b(» of the hiiiiiaii piiiiitiir\ body an* the only remnant of tiie early III I \ »• ii'lU.

I hi' Hiitttiior lobn of the hypophysis is essentially different III Hiii'.iii ii^ wi'll MM in structure from tin* ]>ost<»rior h»be. It i- piodiii-nl b> nil cviiixination from the pcisterior wall of the piiiiiili\c phar\n\,l»ut from that region of the ])harvnx which i- anterior to the |»haryngcal membrane and which therefore bcloii^> to the primitiv(^ mouth-cavity (Fig. GO, p. l;ilj. The out-pocketing of the pharyngeal wall begins in the fourth week, shortly after the rupture of the pharyngeal membrane. The little pouch is the pocket of Bathke. The pouch grows upward and backward toward the floor of the inter-brain and meets the end of the infundibulum. As the pliaryngeal diverticulum lengthens, its stalk becomes a slender duct, which for some time retains its connection with the pharynx. As the membranous base of the skull becomes cartilaginous, the duct begins to atrophy, and finally entirely disappears. In selachians, however, it is retained permanently, establishing thus a connection between the hypophysis and the pharyngeal cavity. AVith the disappearance of the duct the enlarged extremity of the diverticulum becomes a closed vesicle lying now within the cavity of the brain-case, in contact with the end of the infundibulum. From the wall of the vesicle numerous little tubular projections grow out into the enveloping mesodermic tissue, and these, by detachment from the parent vesicle, become closed tubes or follicles. The entire structure becomes converted in this manner into a mass of closed follicles held together by connective tissue, after which event this mass acquires intimate union with the infundibular lobe.

Thus the pituitary body consists of two genetically distinct parts, the anterior lobe being derived from the ectoderm of the primitive pharyngeal or buccal cavity, and the posterior lobe from the ectoderm of the central nervous svstem. The posterior lobe, developing as it does as an evagination from the floor of the inter-brain, is to be regarded as a small outlying lobe of the brain.

AVHiat remains of the cavity of the inter-brain, after its walls have thus developed into the several structures described, is the third ventricle of the adult brain, and the aperture of communication with the secondary fore-brain vesicles becomes the foramen of Monro. Since the lateral walls become the massive optic thalami, while the dorsal and ventral walls give rise to much thinner structures, the cavity of the vesicle is encroached up(m to a greater extent on the sides than from above and below, and hence the form of the third ventricle in the mature condition is that of a narrow vertical fissure between the thalami.

The Metamorphosis of the Fore-brain Vesicle. —

The secondary lore-brain vesicle gives rise to the telencephalon, which includes the cerebral hemispheres and the structures belonging directly to them. As above indicated, this vesicle grows from the anterior wall of the primary forebntin vesicle as a diverticulum which is at first single, but which sfK)n becomes divided into two lateral halves by the formation of a cleft in the median plane (Fig. 147, /6). This cleft or interpallial fissure is the early representative of the longitudinal fissure of the adult cerebrum. The two vesicles remain attached at their bases or stalks with the parent vesicle and communicate by a common orifice with its cavity. The vesicles of tiie secondarj' fore-brain grow in an upward and backward direction as well as laterally, and their develo])ment is so much more raj)id than that of the other vesicles that they soon spread over them and partially hide them from view. It is for this reason that the mass resulting from the fore-bniin vesicles, except their basal ganglia, is known in comparative anatomy as tiie pallium or mantle (Fig. 144).

The relative rate of growth of the cerebral hemispheres is such that in the third month th(»y completely overlie the inter-bniin and bv the sixth month thev have extended so far back as to hide the corpora rjuadrigcniina.

The mesodermic tissue surrounding the developing brain becomes ditlerentiated into the three brain-membranes, which penetrate into the fissure and thereforc invest the vesicles on their mesial surfaces as well as elsewhere. The invaginating layers of the dura mater constitute the ])rimitive falx cerebri.

The metamorphosis of this pair of sacs into the cerebral hemis])heres is broujrht about by three important processes : first, the multiplication of the cells whicli compose its walls to form the masses <»f nerve-cells and fibers of the hemispheres ; second, the formation of folds in the wall whereby are pr<Khiced the fissures which divide the hemispheres into lobes and convolutions ; and third, the development of adhesions within certain areas between the mesial walls of the two vesicles, by which the system of commissures of the hemispheres is produced.

The walls of the cerebral vesicles are at first very thin, consisting merely of several layers of spindle-shaped cells. By the rapid multiplication of these cells, the walls are thick•ened and the cavity of the vesicle is gradually encroached upon until the mature condition of the brain is attained, when the cavity is relatively very much smaller than in the fetus and constitutes the ventricle of the hemisphere or the lateral ventricle. The nerve-cells develop processes or polar prolongations, of which the most conspicuous, the axis-cylinder processes, lengthen out to form the axis cylinders of nerve-fibers. The fibers thus formed are directed away from the surface and make up the white medullary matter of the hemispheres, while the more superficially placed layers of cells constitute the gray matter of the cortex of the brain.

In addition to the cortical or superficial gray matter there are masses of gray matter within the hemisphere, the basal ganglia, which are likewise collections of nerve-cells. Witliin a limited area on the lateral wall of each cerebral vesicle, near the lower margin, the cells undergo excessive proliferation resulting in the production of a large ganglionic mass, the corpus striatum, and of two smaller aggregations of cells, the claustrum and the nucleus amygdala. These basal ganglia are in reality an infolded part of the cortex.

Inasmuch as the cortical matter develops more rapidly, as regards superficial extent, than does the medullary substance, the cortex becomes thrown into folds, forming thus the convolutions and fissures of the hemispheres.

Some of the fissures of the brain are produced by an infolding of the entire thickness of the vesicle-wall so that their presence is indicated by corresponding projections in the walls of the ventricles. Such fissures are distinguished as total fissures. Included in this category are the fissure of Sylvius, which is represented in the wall of the lateral ventricle by the corpus striatum; the calcarine fissure, the dentate fissure, and the collateral fissure, which are responsible respectively for the calcar avis, the hippocampus major, and the collateral eminence of the lateral ventricle ; and the gnst transrerse flseure of the brain, the infolded wall in thia case being very thin and consisting merely of the layer of epithelium which covers the choroid plexus.

The flssnre of Sylvins is the earliest fissure formed and one of the most imjmrlant. At an early period in the history of the secondary fore-brain, there is a region in the lower part of the lateral wall of the vesicle where expansion is loss rapid than elsewhere, this area, as it were, remaining fixed. As the vesicle-wall innnediatoly surrounding thb

8))ot etmtiniies to expaml, n dimpling of the wall is produced, (he depression bcinfj (li'sii;imtc(l the fossa of Sylvius (Fig. 152, S'V The jKirt of the vcsiclo-wall In-hind the fos.'ia advances forwanl and downward to form the future temporal lobe, and thus till- fiwHJi ronu's ti» Ih' siirroiindwl hy a convolution having the form of an incfiniplctt' rinfr, i>|M'n in front — the ring lobe. The llcmr of tlii^ fossi undcinocs very eonsidorable thiekeninj; to form ilie basal ganglia — that is, the corpus striatum, the amygdaloid niielens, and lln" ehuistnim. These structures, most conspicuously tli<' i-orpns striafmn, cniToiioh ujKin the cjivily of iho vesiili', the nucleus caudatus of the

corpus Btriatum bulging intu the floor anJ outer wall of the adult lateral ventricle.

The cortical matter of the floor of the fossa of Sylvius, beiug circumscribed by a groove or sulcus, constitutes tbe

bi. wllh right half of fore-braJn. Dved: lb, CBvilf of inter-brHln; hy, stle of b^p. : Mbr. mid-brain roof; mv, nilil-braliL cuvily ; C wrvbi-lliiin : M. medulla

central lobe or island of Reil, which is subsequently brokea up, by seiMjndnry fissures, into from five to seven email convolutions.

By the extension of the fossa of Sylvius backward, and by the increased gi^owth of the vesicle-wall above and below it, the fossil is converted into the flssnre of Sylvius (Fig. 156, B)y and the island of Iteil is hidden from view. Subsequently the ascending and anterior limbs are added to the chief or horizontal part of the Kssure.

The anterior part of the ring lol>e corresponds with the future frontal lobe, the ]K>sterior part represents the parietal lobe while the lower part of the ring becomes the temporal lobe. A backward extension of the ring lobe produces the occipital lobe.

The cavity of the vesicle is mcKlifiiMl in form and extent eoincidentally with the formation of the corpus striatum and the alterations in the ring lobe. Just as the ring lobe partially encircles the fossa of Sylvius, so does the cavity of i\w. ventricle partially encircle the corpus striatum. An anterior prolongation of the cavity extends into the com|>l<»tc(I frontal lobe as the anterior comu of the ventricle, and iin (»xteusion downward and forward into the apex of the temporal lobe constitutes the descending comu, while the posterior horn is ^nulually protruded into the occipital lobe as ihr latter dcvc](>])s. From the earliest stage, therefore, until I he eoiii])lete(l condition is attained, the cavity of the ventrieli' eoiilonns in a general way to the shape of the henii■'?ph«Te. The a])ertiires of (Mummiuieation between the vesirli-j (>r thi* cerebrum and the eavitv of the inter-brain are the lithr Y shaped foramen commune anterius or the foramen of M«»iito.

The numial surfaces of the hemispheres are much modified ht ehintieirr by the (levelopnicnt here of two total fissures, (hr tiiruato flKHure and the choroid fissure. TIksc ap|)ear in Hie lillh week while the ve^i<*les are >till separate fnun each iiilnr ilnNMi III (heir Ntall\< of attaehnient to the inter-brain, pii«ii it» the development, th(M*efor(', of tile eorpiis callosuni .Old the Cnriiiv. The two lis<ni*e^ lie ejox' to<rother, pandlel Willi iiiili othri* an«l with the niarLrin of the riuir lobe, their r»»iir-.i' ront'nnniiiL: lo ihiit ot'ihe eaviiv t»t'the ventricle. Ik»'jiiiiiMiv ne;ir the anlrrioi* evtreniitv of the brain, ahove the

level of the corpus striatum, they pass backward and then downward and afterward forward to terminate near the anterior extremity of the temporal lobe, thus incompletely encircling the striate body.

The arcuate flssnre is the more peripherally placed of the two. Its anterior portion lies just above the region throughout which adhesions subsequently develop between the two hemispheres, or in other words, above the position of the future corpus callosum (Fig. 154, a./.). This part of the arcu

Fig. 154.— Mesial surface of left fore-brain vesicle of brain shown in Fig. 148 (F6) : /.3/, foramen of Monro, or opening into inter-brain ; o/, arcuate fissure : chj, choroid fissure ; r," randbogen," corresponding to future corpus callosum and fornix; o^f, olfactory lobe.

ate fissure is the sulcuB of the corpus callosum of the mature brain. The posterior segment, that which belongs to the temporal lobe (not present at this stage), is the future bippocampal or dentate Assure. The hippocampal fissure is represented ujion the mesial wall of the descending horn of the lateral ventricle by the prominence known as the hippocampus major.

The choroid fissure or fissure of the choroid plexus, forming an incompI<?te ring within, and parallel with, that described by the arcuate fissure, encircles the corpus striatum more closely (Figs. 154, 155). It begins at the foramen of Monro, and its anterior part lies under the position of the body of the future fornix. It then sweeps around into the tem])oral lobe and terminates near the anterior part of the latter. The fissure of the chon)id plexus, like other total fissures, is an infoMing of the wall of the cerebral vesicle. It presents the l>eculiarity, however, that the infolded part of the wall is extremely thin, consisting of but a single layer of epithelial cells. The pia mater, which everywhere closely invests the surface of the bniin, is infolded with the vesicle-wall, the infolded part becoming very vascular and constituting the choroid plexus of the lateral ventricle. The choroid plexus, although within the limits of the ventricle, is excluded, strictly sjKjaking, from its cavity by the layer of epithelium which still covers it and which has been simply pushed before it into that cavity. Since the epithelial layer is very thin and easily ruptured, the choroid fissure is apjiarently an opening into the cavity of the ventricle through which the pia enters ; in the adult it is called the great transverse fissnre of the brain.

The calcarine Assure, another of the total fissures, develops in the latter part of the third month as a branch of the arcuate fissure. It bulges into the mesial wall of the posterior horn of the ventricle, i)r()ducing the elevation known as the calcar avis or hippocampus minor. Since the posterior horn of the ventricle is developed as an extension of the cavity into the backward prolongation of the ring lobe which forms the occipital lobe, the calcarine fissure necessarily is later in appearing than the fissures above described.

The parieto-occipital fissure is added in the fourth month as a branch of the calcarine, ellecting the definite demarcation between the parietal and occi])ital lobes.

The fissure of Rolando develo]is in the latter part of the fifth month in two ])arts. The two furrows are at first entirely se])arat('(l from each other by an intervening area of cortex. Subsecjuently this part of the cortex sinks l>eneath the surface, as it were, sinec it expands less rajndly than the adjacent regions, and in this way the upi>er and lower limbs of the fissure become continuous. The sunken cortical area is to Ix* found even in the adult brain as a deep anneetant gyrus embedded in the Kolandic fissure at the position of its superior genu. TIk^ development of the fissure of Kolando effects the division betw(HMi the fnmtal and jwirietal lobes.

The collateral fissure appears in the sixth month as a longitudinal infolding of the mesial wall of the hemisphere below and parallel with the hippocarapal fissure. Being a total fissure, its presence affects the wall of the cavity of the vesicle, producing the eminentia collateralis. At about the same time the calloso-marginal Assure . makes its appearance, and this is morphologically continuous, through the medium of the post-limbic sulcus, with the collateral fissure (Fig. 157). These three fissures constitute the peripheral boundary of a region of the mesial wall which is known in morphology as the falciform or limbic lobe.

The longitudinal Assure in the early stage of the growth of the cerebrum separates the two vesicles from each other except at the place where they are attached to the inter-brain ; here the two sacs are united by that part of their common anterior wall which is immediately in front of the apertures of communication with the inter-brain and which is called the lamina terminalis.

The development of adhesions between the mesial surfaces of the hemisphere vesicles throughout certain definite areas marks the beginning of the corpus callosnm and the fornix. The fusion of these areas begins in the third month in the region corresponding to the anterior pillars of the fornix, the septum lucidum and the genu of the corpus callosum ; in the fifth and sixth months adhesion occurs in the position of the body of the fornix and of the body and splenium of the corpus callosum.

Although the central white medullary matter of the cerebral hemisphere is covered almost universally by the cortical gray matter, there is a limited area of the mesial surface from which the gray matter is absent, leaving the white matter ex]>osed. The area of uncovered white matter has the form of a narrow band, which begins at the base of the hemisphere, in front of the opening into the inter-brain, extends upward along the anterior wall of the inter-brain, then passes backward along its roof and curves downward and outward behind, and then forward under it, to terminate at the front part of the temporal lobe. Thus this white band, which is known as the fimbria, and which represents the lower mesial edge of the hemisphere, almost encircles the inter-brain. The fimbria runs between the arcuate fit^sureand the fissure of the choroid plexus (Fig. 155, /). It holds such a close relation to the lat

F[o. IM.— Mu'sl«l Bnrftcc of left hcmlsphcp;, hmln of fi'tim of three months (ciilurRiid) : /.. fiirnii: r.r.. beginning of u>rpus cHlluiam; c.rl., partot vuriiui atrtstiiiii iirelilnB uruiind fmra of Sylvius ; a/., unttrinr. mill nj./i.. |HiitvrU'r parti of urrnauj Huiin? ; rhj., i-l]i>ri>id Usrun.', the coni'ui li.v lH-tH'i'i.'ti u'liii'li und the corpni klrlatnin ucComniudatLii the luUT-linin, which hna Ik'vii ivmiirtil. The Itiaare U ■icL'Upiiil by the pla luutuc.

tor fissure, Iwiiig placet! on its ]>erii)licr.il side, that it constitutes the e<lge of the apjtaroiit o|Htniiig into the cavity of the vesicle thi-ough wliiuh the piji niiiter, iKiiriiig bloofl- vessels, is reflctrted iuto the interior, and which, as pointed out above, is the tniiisverM' fissure of the Itniin. The <i|ieuing is only a])piir(^nt, however, since tlif wall is still iinlirokfii, although reduced to ii single layer of ejntltflinin. The pia mater, forming, with its blood-vessels, the cliDniiil plexus of tlic lateral ventri.-Ie, pushes the layer "f cpirhellnin before it, and althoM^di the |)lexns is s;ii«l to be within the eiivily of the ventricle, it is still covtirod by the layer of epitliclinm, the ependrma, whifh lines that oiivity.

Thi' part of (he (iuibria that ini mediately overlies the roof of the inter-hrnin Iieeonii's iiitinmtely uniteil, as noted alx)ve, with the eorri'sponiling jiarl of the titiibria of the other heniis|ihen>, these fused portions of the two limhri;e forming a flat tnangular sheet, the body of the fornix. the anterior and IKisti'i'ior portions of the fimbria, wich diverge from the moilian plane, represent res|Mitiv<'ly the anterior ami posterior limbs of the fiiniix.

Noting the relation ->r tlu^ anterior part of the fimbria to till- a]>or(ur(Mif eonniinnii'ation between the inler-brain and the cereijnil vesicles, it becomes apparent that the anterior pillar of the fornix forms the anterior and n|»per Iwuiidarj' of

the foranifQ of Munro. When, further, one considers the relatiou uf the fimliria to the apparent oi>ening into the ventricle, through which the pia mater i« invaginated (the transverse fissure), it is explained why the edge of the fornix appears as a narrcjw white band, not only as viewed from within tile ventricular cavity, Imt tiho in a nit-Mal section of the brain (Fig. 156, C).

t^o. liiK.—Fctal tiriiln at thr \ieg\xm\ng of Ihe tiiihlh moiitli <MlliaIkovlm> : A.iiiperloi, B. Inderal, C, rnvslnl aiirfaru: K. tliaiiK of KoUndo: prf, ■■reecnlral fi«Buw; S|f, HylirlanHwure: inip, lntBrp»rletiil llHiiin.'; jhw, parfet(ww«lpil«l(i»aurB! pU, psratlel tiiuurv: eailra, calloiomirglnnl Buure: u'T, unoui : cale. culCBTine

Another important region of fusion of the opposed mesial surfaces of the hemispheres is that corresponding lo the future corpus calloBum Throughout this area the liemispheres closely unite with each otiier The line of fusion begins at the Imses of the vesicles, some little distance in front of the anterior parts of the fimbriie (Fig. 155, f-c). and after passing upward and luruird, curves horizontally backward kward I

in close relation with the fused portions of the fimbria?, now the body of the fornix. The atUiesiou begins at the anterior part in the third month, and atfects the regit)n of the body and gplenlum of the future corpus callosum in the fifth and sixth mouths. Fibers penetrate from one hemisphere to the , other throughout this zone of contact, intimately uniting the cerebral hemispheres. The corpus callosum is therefore & great commissure lietween the two halves of the cerebrum, and is necessarily composed of fibers having a transverse 1 direction.

While the back part of the corpus callosum lies over the body of the fornix and is in close contact with it, the front part of the body of the corpus collusum, as also its genu or | curve and its rostrum or ascending part are at some distance ( from the front parts of the fimbrite. In other words, while the j great longitudinal fissure extends at first to the bases of the cerebral vesicles, this fissure is made relatively loss deep by the adhesions which occur between the mesial walls and which result in the development of the corpus callosum ; and the space below the anterior part of the corpus callosum, between it and the anterior parts of the fimhriie (Fig. 1.56, C), is an isobtied part of the great lonrjitudinal fissure. This space is bounded on either side by thatjtart nf the wall of the corres- , ponding cerebral vesicle or hemisphere which is limited above | and in front liy the corpus collusum, and behind by the anterior part of the fimbria or anterior limb of the fornix. The ' space is the so-cniled fifth ventricle of the adult brain. The | circumscribed parts of the mesial walls of the hemisphei which form the lateral walls of the space, together constitute ' the Beptum lucidum. The jtarts of the hemisphere walls that ' become the septum lucidum do not participate iu the procesa ' of fusion mentioned above. Their surfaces are iu contact) [ however, and do not develop the typical gray cortical matter, I such OS appears elsewhere ujKin the surface of the cerebrum. J Cortical gray matter is produced here, but only iu radi- 1 mentary form.

From what has been said, it will be seen that the t layers of the septum lucidum are circumscritKid and opposed.

parts of the mesial walla of the hemispheres ; that the fifth ventricle is not a tnie ventricle but an isolated part of the longitudinal tiggure having no connection whatever witli the system of ventricular cavities ; and that this .so-called ventricle is not, like the true ventricles of the brain, lined with ependyma, but with atrophic gray cortical matter.

The limbic lobe has been referred to as that part of the mesial aurf'ace of the hemisphere which is circumscribed by the calloso- marginal fissure, the post-limbic sulcus, and the collateral fissure. It is limite<l centrally by the fissure of the corpus callosum and the hippocampal fissure, which are represented in the fetal brain by the single uninterrupted arcuate fissure. Hence the limbic lobe would include the gyrus fornicatus, the isthmus, and the gyrus uncinatua, which constitute morphologically a single ring-like convolution. Schwalbe, however, includes with this so-called limbic lobe all the surface of the mesial wall of the hemisphere included between the arcuate fissure and the fissure of the choroid plexua (Fig. 154), designating it the folciform lobe (Fig. 157).

The falciform lobo therefore consists of two ring-like convolutions, one within the other, the two l)eing separated from each other by the arcuate fissure (the adult callosal and dentaf« fissures) and being limited centrally by the fissure of the choruiii plexus (the great transverse fitisurc of the adult brain). While the outer of these concentric convolutions — the limbic lobe of Broca — develops into the fornicate or callosal, the isthmian, and the uncinate gyri, the inner ring differentiates but slightly, its cortical matter remaining atrophic. The atrophic condition of the cortex here is associated with those adhesions between the mesial walls of the hemispheres that result in the formation of the corpus callosum and the septum lucidum. By these adhesions the continuity of the inner concentric convolution is broken, and it is therefore represented, after the development of the corpus callosum, by the atrophic gray matter of the septum lucidum, by the gyrus dentatus, and by the lateral longitudinal striae on the free surface of the cor])us callosum, the latter being an atrophic or rudimentary convolution. Since the transverse fissure of the brain is the centric boundary of the ring, the fornix is also a part of the falciform lobe. To sum up, the falciform lobe includes the gyrus fornicatus, the isthmus, the gyrus uncinatus, the lateral longitudinal striae or taenia tectae of the corpus callosum, the gyrus dentatus, the lamina* of the septum lucidum and the fornix.

The olfactory lobe or rhinencephalon is an outgrowth from the vesicle of the cerebral hemisphere. Its development begins in the fifth week by the pouehing-out of the wall of the vesicle near the anterior part of its fioor (Figs. l-t7 and 149). This diverticulum, which contains a cavity contiinious with that (»f the vesicle, grows forward and so<ni l)ecomes somewhat clul)-sha]>ed. In the selachian.'^ (.sharks and dog-fish) the projection attains a great relative size, the olfactory lobes in these animals being one of the most eonsj)icuous ]uirts of the brain. In all mammals except man it is well developed, and in the horse its cavity persists throughout life. In man the cavitv soon becomes obliterated and the lobe itself in part aborts. The ])rotru(le(l })ortion, becoming more distinctly club-shaped, differentiates into the olfactory bulb and the olfactory tract, the ))osition of the original cavity being indicated by a more or less central mass of neuroglia conspicuous in cross-sections of those structures. The proximal portion (»f the olfactory lobe is represented in the adult human Urain by the gray matter of the anterior perforated lamina {or space), ami by the trigonuin ol facto riiiiii and the area of Broca, as well aa by the inner and outer roots of the olfactory tract (note olfactorj- lobe of dog's brain, t"ig. 156).

Fig.— Buc of dogabialn: al., o\lar\otj lube: a.ji,»., rcxiin corrcipandiiiK M r perforaled spa<-c. which la Incluiled in the ulllictorv l<ibc ; /.S.. IliiBurt nT i; a.h., hlppooampal ((>tus. deTelopvd to a Rn-iitor rtpRree than In human 1., leellonal siirCice of oltactoty lobe: m.. olltictury sulcus.

Because of the relation of the place of evagination of the olfactory lobe to the fossa of Sylvius, it happens that a |>art of this lobe, the anterior perforated lamina, is i^ititatcd at the commeneeincnt of the fissure of Sylvius and that it is in eoiithmity with Iroth extremities of the ring lobe; hence, the olfactory lobe Is connected with both extremities of the falciform lobe. To express it in the language of human anatomy, the outer or lateral root of the olfactory tract is connected with the gyiuB uncinatUB, while the inner or mesial root'may be traced to the fore part of the gyrus fomicatns.

After what has been said, the reader need scarcely be reminded that the olfactory bulb and tract, often erroneously referred to as the olfactory nerAe, are parts of a lobe of the

iirain, a lobe which in man is rudimentary but which in all o(h(M* mammals is well developed.

Tahulaied Rijnim^ of ihe Derivatives of the Brain-Besides,

Hkain VKtiK'l.KM. AfliT tiralu

Ililidbrnln vuhIcIo.

Mill tiruin VVblcie.

IlltlT

hritiii

Floor.

Medulla oblongata.

PonH Varolii.

Roof.

Tela choroidea inferior.

Lateral walls.

Inferior peduncles of cerebellum.

Hiiroiidary fi in* bruin Vfnirlt*.

Peduncles of cerebrum. Posterior perforated space.

(N)rpora albicantlii. Tuber cintTcum, infundlbulum, and imrt of hypojihvHis. Uptic ehiiiHm.

AntiTlor |K'rfonittMl lamina. CorpUM 8triiitum. Island of \W\\. Olfai'tory lolic.

Po8terit)r medullary velum. Cerebellum. Anteri(»r medullary velum.

Posterior part of toKnientum.

Corpora quadriuemina. I^amina quadrigemina.

Pineal l>ody. Posterior commissure. Epithelium of velum interpositum.

Middle and superior peduncles of cerebellum.

Brachia. Internal geniculate bodies.

Optic thalami.

Convolutions of cerebral hemisplieres. Corpus rallosum. Fornix. iSeptum lucidum.

Cavitt.

Fourth ventricle.

Aqueduct of Sylvius.

Third ventricle.

Lateral ventricles.

TIII2 DEVELOPMENT OF THE PERIPHERAL NERVOUS SYSTEM.

'I'lif (l<»v<*l<>|)m<'nt of the pcri])heral nervous system is still iiiV(»lvr<l ill some ilejxroe of obscurity. In general terms it nmy Iw .slal4*(l that tin* ])eri])lieral nervous apparatus is derive<l as an e\t<>nsion of the central cerehro-spinal axis.

In the ease of the spinal nerves, e^'ich nerve-trunk is com* poseil of hoth motor aiul sen.<orv fihers, the former being in continuity with the spinal cord tlirough the medium of the anterior (»r motor roots, and the latter through the posterior or sensory roots, ea<'h sensory root ))o.^.sessing a ganglion. The cranial nerves exhibit a less n^giilar composition. While the trigeminal nerve, for e.xam|>le, arises by two roots, after the manner of a spinal nerve, some others correspcmd in relative {N)sition and in mod(» of d<»velopinent to the ventral or motor r(M>ts of th(» spinal nerves, and still others are equivalent to the Hi»nsory spinal roots.

The deTelopment of the sensoir nerve-Sbers ib d^jH-udent upon and is preceiicd liy tliiit of the ganglia of the posterior roots of the tspinal nervi'.=, and nf several ganglia of the head rt'giiin which are related tu the development of certain of the cranial nerves. Hence the consideration of the genesis of

Firi. IM.

oritr Rnbl). The

primitive ergments vn atlLt conneL'Icd wilh Ihe reiaiiluluK porllnn genn-ltiTer. At the regiuu of tmnnltlon lliere ie to be ii

muHclc-plato ut the primiLlvc x gemi-Uyer ; pmi. i»ri«Ul, imi6, vlwx-ral mldillB Ujrpr. 8, crotMCClion through a Ilunl emhryo (nflvr Sagvuii'hl) : m, spinal cori] : tpa. lower thickened part of (he nuurul ridge: ipp'. Its Dpptr allcnuated part, which Ii conllnuoiu with the Toorof the Deural tube : ui. primitive aegmunt,

the ganglia niiist precede the account of the growth of the sensory nerve-fibers.

The oriKin of the ganglia is connected with the early history of the evoliilion nf the neural tuhe. Just after the sides of the medullary plate {vide p. 279) have united with each other to form the neural tube, there appear two ridges of cells between the tube and the epidermis, one on each side of the raphe or line of union of the sides of the tube. Thene ridges are the neural crests (Fig. 159). They first appear in the region of the hind-brain and advance from this point both headward and tailward. The ganglia develop from these neural crests. The cells of the neural crest are usually described as growing out from the neural tube, though according to His it is probable that they originate singly from the ectoderm.

Tlic mass of cells composing the neural crest grows out\vard and then ventrad along the Avail of the neural tube, and very soon undergoes segmentation into a number of cellmasses which are the rudimentary ganglia. In the spinal region the number of segments corresponds to the number of future spinal nerves. In the head region there are four segments. These latter, the cephalic ganglia, will be referred to sul)se(iuently.

The segmentation of the neural crest corresponds in the main witli the sogmcntation of the ])araxial plate of the inc.-odcrm, whereby the myotomes are ))ro(luced, and ench segment lies upon the inner side of :i myotome. The connection of the segments with the neural tnl)e beccmies reduced in each case to a slender strand, the point of continuity of which with the tube is shifted farther awav from the median line, as (level(>])nient proi^resses, to eorresptmd with tl)e dorsolateral position of tiie sensory nerve-roots in the mature condition.

theeells of the tranirlia ae<|nir(» eaeli an axis-evlinder process and a (h-ndrite or |)n>toj)la>iiiie process, beeoniing thus bijKjIar e<'lls. Tli«' axons or axis-eylinder |>i'oeesses make their way into the spinal e<)nl — in the ease of* the spinal gauiilia — eunstitnting ilui> the dorsal nerve-roots, and ])nrsne their eour>e within tiie eord as the e(>lnnins of (loll and Bnr<la('h. The (hndrites, eon>titntini:' llie distal ]>ortions of the dorsal roots, join tlir ventral r(K)i< (»ii the distal si<le of the iranulia and lM'<'onie liie sensory nerve-fibers of* the spinal nerves. Allhonirji these two proe«.*sses grow out from opposite sides of the cell, the further growth of the cell is such that both processes are connected with it by a common stalk, thus producing the cell with T-sha|)ed process so characteristic of the spinal ganglia. Thus the ganglia are made up of cells which are interpolated in the course of the sensory nerve-fibers, and these cells may be regarded as having mignited from the developing cerebrospinal axis, or, if the view of His be acc^ptwl, from the region of the e^ctoderm from which the tube originates, their connection with the axis l)eing maintained by the gradually lengthening out axis-cylinder process.

The development of the motor nerve-fibers differs from that of the sensory. These fibers, or at least, the axis cylinders of the fibers, are the elongated neurits of nerve-cells of the spinal cord and brain. The neuroblasts of the thickened neural tube, as they become fully differentiated nerve-cells, migrate from their central position into the mantel layer, or superficial stratum (Fig. 140). On the distal side of the nucleus of the cell, the protoplasm first becomes massed and then lengthens out to form an axis-cylinder processor neurit, which in all vertebrate animals grows out from the cerebrospinal axis to form the axis-cylinder of a motor nerve-fiber.

Although, in the wise of the spinal nerves, the motor and sensory fibers are separated from each other at their origin from the cord, they soon intermingle to constitute a spinal nerve-trunk. In certain lower types, as cyclostomes and amphioxus, the motor and the sensory fibers permanently pursue separate routes to their ]>eriphenil distribution.

The envelopes of the nerve-fiber are acquired at a relatively late period. The appearance of the neurilemma precedes that of the white substance of Schwann. The neurilemma is derived from the mesmlerm. The cells of the latter apply themselves to the nerve and, penetrating between the fibers, become arranged as an enveloping layer upon each axis cylinder, ultimately forming a complete sheath, the neurilemma. The persistent nuclei of these cells, scantily surrounded with protoplasm, constitute the nervecorpuscles of the neurilemma. The medulla, or white substance of Schwann, is formed at a considerably later period within the neurilemma. The ileixwit of the medullary sheath 1 varies as to time lor (Jifferent groups of SIhts — although the ] time is constant for each groti|j — unJ proceeds always in a direction away from the cell from which the fiber originates, or, differently expressed, in the direction in which the fiber eonveys impulses. Thus, in the spinal cord, groups of afferent fibers may be distinguished from those that are efferent hf observing the direction in wliich the medullary sheath develops — that is, whetiier tlio sheath appears first at the upper end | of the fiber or at the lower end.

The cranial nerve-flbers in their development follow in the \ main the same general principles that govern the growth of the iipina! nerves. That is to say, the motor fibers grow out 1 as extensions of the axis-cylinder processes of nerve-cells of i the cephalic jmrt of the neural tul)e and the sensory fibers \ proceed from the cells of outlying ganglia, or in the case of at least one nerve, the olfactory, from infolded and highly I specialized ceils of the ectoderm.

The cephalic ganglia, four in number, have been referred I to as resulting from tiie segmentation of the head-region of ] the neural crest. As previously stated, the neural crest I begins to grow first in the region of the hind-brain and j extends from this point both forward and backward, occupying a iwsition upon the roof or dorsal wall of the hind-brain. \ The part of the neural crest belonging to the head-region i then divides into the four masses or head-giinglia which are I designated respectively the first or tiigemmaJ, the second or ] acosticofadal, the third or glosBophanrngeal, and the fourth ; or vagal, ganglia.

The trigeminal ganglion, which is very large, becomes di- i vidwl into a smaller anterior (M>rtiou, the ophthalmic or ciliaiT ganglion, and a. larger posterior segment, the tiigemlsal j ganglion proper. These two become widely so|mrated durin^J llic pnigres.'i of development, since they constitute respeo-J lively the later ciliary and aasserian ganglia, the ciliary I ganglion belonging to the ophthalmic division of the fifth I nerve, while the trigeminal belongs to the sui^-rior maxillary J division and the sensory part of the inferior maxillary divi- J

<n of the fifth. Their nerve-cells give rW. to the sensory fibers of these trunks in the same manner that the cells of the spinal ganglia produce the sensory fibers of the spinal nerves.

The acusticofacial ganglion, afler its migration from its original position on the dorsum of the hind-brain, lies just in front of the otic vesicle. This ganglion subsequently divides into the facial and the acoustic ganglia. The facial ganglion, the geniculate ganglion or intumescentia ganglioform is of the facial nerve, situated in the facial canal of the temporal bone, although described as a ganglion upon a motor nerve, the facial, is, in reality, connected mainly with the pars intermedia, a bundle of sensory fibers issuing from the nucleus of origin of the glossopharyngeal nerve. It is equivalent therefore to a spinal ganglion.

The acoustic portion of the acusticofacial ganglion divides still further to become the ganglion on the vestibular part of the auditory nerve, and the ganglion spirale of the cochlear division of the auditory, which latter is situated in the spiral canal of the modiolus. It is considered probable that the lateral accessory auditory nucleus, which is connected with the cochlear fibers of the auditory nerve and lies on the outer side of the restiform body, is also a part of the acoustic ganglion. From the cells of the vestibular ganglion, which is situated in the internal meatus, centrifugal fibers develop to form the vestibular nerve, while other centripetally growing fibers become the ventral or mesial (vestibular) root of the auditory nerve. The cochlear ganglion in the same way gives rise to the cochlear branch of the nerve and to its dorsal or lateral root. Thus the auditory nerve and its ganglia correspond respectively to the sensory root of a spinal nerve and to a spinal ganglion.

The third cephalic ganglion becomes the ganglion of the glossopharyngeal nerve, undergoing segmentation to form the upper or jugular and the lower or petrous ganglia of this nerve, while the axis-cylinder processes of its cells lengthen out to become the sensory fibers.

The fourth cephalic ganglion similarly becomes the two ganglia of the pneumogastric nerve and gives rise to its sensory fibers.

From what has been said, it will be apparent that the cranial nerves develop in a far less regular manner than the spinal nerves, and that consequently their trunks consist in some cases of only sensory fibers, in other cases of only motor fibers, and in still others, of both varieties. Typically, each cranial nerve would have a dorsal sensory root with a ganglion, and two motor roots, one lateral and the other ventral. But by the suppression of one or two of these typical roots there will be produced a nerve, for example, representing only the ventral root, as the sixth and twelfth nerves, or a trunk containing sensory and lateral motor fibers, as the vagus, or a nerve consisting solely of sensory fibers, as the auditory.

By way of recapitulation the cranial nerves may be briefly considered seriatim :

First Pair. — The olfactory nerve-filaments grow centripetal ly from the olfactory epithelium of the nasal mucous membrane.

Second Pair. — The optic nerve is not a true nerve (see Chapter XVI.).

Third Pair. — The oculomotor nerve represents a persistent lateral motor root of the first head-segment (the ophthalmic division of the fifth nerve being the sensory root of the same segment).

Fourth Pair. — The trochlear nerve represents a lateral motor root and belongs to the second head-segment.

Fifth Pair. — The trifacial or trigeminal nerve, containing sensory and motor fibers, represents a persistent lateral motor root and a dorsal sensory root. The ophthalmic portion of the sensory root belongs to the first head-segment, while all the remaining fibers, with the fourth nerve, are assigned to the second segment.

Sixth Pair. — The abducens develops as a ventral motor root and belongs to the third and possibly to the fourth segments.

Seventh and Eighth Pairs. — The acusticofacialis nerve, or the facial and auditory nerves, develop as a single nerve with ^veral roots. The auditory nerve and the sensory fibers ^^

the facial — that is, the pars intermedia — correspond to a dorsal sensory root, the division of the acusticofacial ganglion into the several ganglia of the auditory nerve and the geniculate ganglion of the facial accounting for the division of the root into the auditory trunk and the ]xirs intermedia. (The sen* sory tibors of the facial pass off through the chorda tympani to go to the tongue as special-sense fibers.) The motor fibers of the facial develop as a lateral motor root^ originating from c-ells in the ventral zone. These two nerves, with the sixth, belong to the third and possibly to the fourth head* segments.

Xinth Pair. — The glossopharyngeal nerve, made up largely of s(»ns()rv fibers?, re[)resents a dorsal sensory root and a lateral motor root, the fib(»rs of which latter grow out from cells in the dorsal ])art of the v(»ntral zone of His, the later micleus ambigmis. It belongs to the fifth head-segment.

Tenth Pair. — The vagus develops in the same manner as the glossoj)harvngeal.

Eleventh Pair. — The sj)inal acoessorv represents in part motor spinal roots and in [)art probably the lateral motor and dorsal scnsorv roots of the cranial nerves.

Twelfth l^iir. — The liy[)ogIossal develops as the ventral motor roots of several segments, being identical in mode of origin with the anterior roots of the sjnnal nerves. This nerve and the vati^us b(*long to the head-sc»gments from the sixth to the tenth inclusive.

THE DEVELOPMENT OF THE SYMPATHETIC SYSTEM.

There are two views as to the origin of the sympathetio system. One theory, based uj)on the investigjitions of Paterson, is that the gangliated cord of the sym[)athetic is differentiated from mesodermie cells, the eell-eord thus formed acquiring, secondarily, coinieetions with the spinal nerves, and ])res<Miting still hiter the enlargements which C4mstitute the ganglia.

The more genendly accept(»d vi(?w, based upon the researches of Rilfour and the later work of Onodi and His, is that the Bsrmpathetie ganglia develop as offshoots from the ventral extremities of the spinal ganglia. Each little mass, which has budded off from a spinal ganglion, moves somewhat toward the ventral surface of the body, its bond of union with the parent spinal ganglion being drawn out to a slender cord, the representative of the future ramus communicans. Each primitive sympathetic ganglion sends out two small processes, one growing tail ward from its lower extremity, and one in the opposite direction from its upper end, the approaching processes from each two adjacent ganglia meeting and uniting and thus secondarily establishing the connection between the different ganglia of one side of the body and forming the gangliated cord of the ssrmpathetic. From these ganglia migrating cells probably pass out to develop into the secondary ganglia of certain viscera, as His has shown to be the mode of origin of the ganglia of the heart.

THE CAROTID BODY, THE COCCYGEAL BODY, THE ORGANS OF ZUCKERKANDL.

In connection with the sympathetic system may be mentioned the " carotid gland," or glomus caroticus, or intercarotid ganglion, found at the bifurcation of the common carotid artery ; the coccygeal body or " gland," Luschka's ganglion, found at the lower extremity of the coccyx in relation with branches of the middle sacral artery; and the organs of Zuckerkandl, found in later fetal life and for a short time after birth at the origin of the inferior mesenteric artery.

These structures present features in common with each other in that they are made up of knots of blood-vessels intermingled with collections of cells, among which are numerous chroviaffine cells such as are found in the medulla of the adrenal body and in the sympathetic ganglia ; and in the furthor fact that they are penetrated by sympathetic nerve-fibers.

That the cells of the carotid body and of the organs of Zuckerkandl are derived from the adjacent sympathetic ganglia has been established, but whether these bodies are for that reason to be classed as nervous structures is as yet uncertain.

+++++++++++++++++++++++++

CHAPTER XVI. THE DEVELOPMENT OF THE SENSE ORGANS.

In the organs of the .senses we have to do with peripheral nervous mechanisms of greater or less degrees of complexity, the essential elements of which are elalK>ratelv modified or specialized neiiro-epithelial cells. These neuro-epithelial structures are specialized cells of the ectoderm, derive<l from it either directly, hy the infolding of patches of ectodermic epithelium, as in the case of the olfactory cells, or indirectly, by growth outward from the central nervous system, as in the case of the retina. The organs of the sense of touch, the tactih' corpuscles of the skin and mucous membranes, are distributed somewhat irregularly, while such highly Sjweialized struc^tures as the organs of the special senses of vision, hearing, smell, and taste are provided with special protective and accessory apparatuses.

THE DEVELOPMENT OF THE EYE.

It will perhaps larilitatc th<* eoinpreliension of the general principles involved in the (Ifvclopincnt of the eye if its function as the organ of vision is. krpt in mind, and if, therefore, the retina and the o|)tic nerve are recognized as the essential parts (»f the organ, and the other structures as accessories. The retina an<l the optic nerve are an outgrowth from the brain, the rod- ai]<I eone-visual <'ells of the former being epithelial cells so specialized as to serve as j)ercipient eh-ments, while the (»j)tic nerve-fibers are the (-onducting medium. To alh)Wof tlu' penetration and refra<'ti<m of the ravs of liirht, the (»verlving epich^rmis differentiates into a transparent and refra<*tive medium, the crystalline lens, and the necessary prote<»tion an<l means of nourishment are provided by the other constituent of the eyeball. Further protection is furnished by two folds of modified skin and subcutaneous tissue, the esrelida, and lastly for the lubrication and still further protection of the exposed part of the eyeball, there is formed still another set of accessory organs, the l&crimal appuratiu.

The first step in the development of the eye is the growth of a diverticulum from the side of the primary fore-brain vesicle (Fig, 160). These optic evaginations are qnite large

II of two-d«j- chlck-embryn ; B. bmln of huni»n embrro of Oiowii the development of tbe opllc leslclva and bnlD-resl< h. inter-brain; ob, optic veulcles.

as com{)arGd with the h rain- vesicle. They begin to be evident even before the neural tube is completely closed. As the attached part of the diverticulum expands less rapidly than the distal ]>ortioii, the evagination soon assumes the form of a sac or vesicle, the optic vesicle, connected by a hollow stalk with the primarj' fore-brain. When the secondary fore-brain vesicles gn)w out anteriorly from the primary vesicle, the region of the latter that Itecomes in consequence the inter-brain is the part to which the stalk of the optic vesiele in attachetl. Hence the optic vesicle is an appendage of the inter-brain or thalamoncephalon and its point of attachment to the latter Is at the lateral iwrt of the base, in front of the region of the infundihiihim (Fig. 147, A and C).

The optic vesicle expands laterally and dorsally until.it lies immediately lieneath the epidermis, forming a prorai

nenee on the side of the head (Fig. 62). The ectoderm at the point of omtaet with the optic ver^iele becomes thick* ened and depressed, the dilTerentiation of this lens-area being the >tartin^ p«.»int of the erystaUine lem. The depressed |iatoli of ectixlemi, sinking more <k»eply, is converted into a sac, the lens-vesicle, the o >nnt*etion of which with the surfacecells is <mM\ l«»st. The ilistal wall of the optic vesicle, upon coming into contact M'ith the lens-vesicle, undergoes invagination, this wall sinking in until the cavity of the vesicle is ahno-t obliterated. Thus the vesicle is converted into the druilile- walled optic cnp, the o|K'ning of which looks laterally toward the surface of the head, and is occupied by the leusvesiele.

The invaginated wall of the vesicle — thatis, the layer nearer the c<*nter of the cuji — becomes the retina, except its pigment-layer, the latter resulting fn)m the outer layer of the cup. The stalk <»f the cup l)ecomes the optic nerre. The surroimding mesodermic tissue grows into the openings nrferred to above, and gives rise to the vitreous hnmor, while the mesodermic cells that closely envelop the optic cup produce the uveal tract and the sclera and cornea.

Having tra<*ed briefly the develripment of the organ, its sevcnil parts may now bo considered in detail.

The Retina and the Optic Nerve. — These two structures, as stated abov<*, an; directly derive<l from the optic vesieh; and its stalk.

To rep4*at, for the sake of continuity, some points already mentioned, tin; optic vesicle grows forth as a diverticidum from the side; of tin? primary fore-brain vesicle, its appearance beting foreshadowed by a lateral bulging of this vesicle even before the neural canal is com[)letely eloswl. When the primary fore-bniin vesicle divides into the secondary fore-brain vesicles and the vesicle of the inter-brain, the regir>n of origin of the optic vesicle falls to the latter, the jM»int (»f attachment being at the outer edge of the base of the vesicle in front of the infundibular evagination. The optic nerve is to be regar<le<l th(Tefore as springing from the inter-brain or thahunen<*e|)halon.

Fig. 162.— Three BUMtmlvc sMgca of dsTelopnient of the eye, showinE fOmt.tlon or aeenndary nptic cup and crrilaltlne leiu in human embryos or 4 mm. < J), fl mm. (B), &nd H mm,(C]. (Tonnieui): a, a, primltlTe optic veilclei; b, extern *l Uyer of neciuidary opllc cup (fliture pfttmenl-Uyor of rellna) ; c. Inner layer of cnp (re linn proper) ; il, lens-pit llhlekened and depreued eatudena):

diattfly under the epidermis, separated from it by nnly a thin layer of embryonal connective tisBtie. This lateral position of the optic vesicles is characteristic of the early stages of development. After the end of the first month the eyes gradually move forward and downward toward their permanent position, which is approximately attained probably early in the third month.

Shortly after the fourth week the distal or lateral wall and the under surface of the optic vesicle l>ecome invaginated. The invagination begins when the vesicle comes into contact with the lens-vesicle (Fig. 162). When the infolding is complete, the vesicle has l>econie the secondsiy optic cap, which latter consists therefore of two layers, an inner and an outer. The month of the cup, which faces away ffom the niMliaii plane of the head, is occupied by the lens-vesicle. Since the under surface of the vesicle ]>artici))a(es in the invnginating pniee.'is (Fig. 163) there is also in this wall of the enp an ajwrture, which is known as the choroidal flsmre. The invagination likcwi.sc affects the under siirfatre of the tubular .-^lalk of the vesicle sii that it is c<mvertc<l into an inverted (loultle-Iaycred trough. These invaginations bear an important relation not only to the further nK'taniorphosis of the optic vcsieli" lunl its stalk into the retina and thf.iptic- nerve, but also to the (levc4o|)nnTit iif the vitreous body ami of llic r<-iiti-al artery of tlic n.'lina. TliMs, the vitreous binly is PrmIiiwiI in yurX at !ca>t by the Kie^iilcrmic tissue that finds access to the cui) tlin.Li<rh llic ehoniidai fiTisure, and tlic arloria wntralis n-i'mw is di'v.IojK'd in the vascular ■ tissue iliat invagliiates -uriaee of the stalk of

leM-lcl

the I the '

si.-Ie.

gnuhially 1-oiitraeis after ilic enti-!in<-e of the niescRlerni, and in the liist motitli of fetal life it entirely closes. Tiie mouth of ilie o]Hie cup einlinices tite lens, its rim being always on the ilislal side of, or su]>crlii-ial to, that .-itructure. This iijieiiiiig repiTM-ntf* the pupil of later stagi-s.

The further metamorphosis of the optic cup includes alterations peculiar to each of the two layers and also to the different regions of the cup. The mouth of the cup contracts somewhat by increased growth of the wall, and thus there is a zone bordering this orifice which is anterior to the lens, holding the same relation to the latter body that the future iris holds. A second zone corresponds with the periphery of the lens, while a third region, the fundus of the cup, includes all the remaining part of its wall.

The flmdas of the cup undergoes much greater specialization than the other regions. The outer layer of the cup remains thin, consisting of a single layer of cells which assume the cuboidal form and become infiltrated with pigment-granules. This forms the pigmenlrlayer of the retina. The inner lamina of the cup thickens, by the multiplication of its cells, and soon consists of numerous spindle-shaped cells. The thickened fundus is marked off from the zone that surrounds the periphery of the lens by a slight groove which corresponds in position with the future ora serrata. These early spindlc-cells give rise to two kinds of elements, the stroma of the retina, or Miiller's fibers, and the various nerve-cells, including the highly specialized rod- and cone-visnal cells.

The principal sustentacular elements, or Miiller^s fibers, like the spongioblasts of the neural tube, are radially arranged and extend throughout the entire tliickness of the retina. Their inner expanded extremities, in close contact with each other, form the inner limiting membrane, while their outer ends, in the same way, constitute the outer limiting membrane, which latter is in contact with the pigmentlaver. The stroma of the retina receives a small contribution from the mesodermic tissue, which grows into it through the choroidal fissure to furnish the vascular supply.

Of the nerve-cells, those near the pigment-layer undergo great alteration in form and become the sensory epithelium — that is, the rod- and cone-visual cells. At first these lie entirely internal to the external limiting membrane, which separates them from the pigment-layer. After a time, liowever, processes grow out — that is, away from the center of the eyeljall — and ]>erforate the external limiting membrane to i»eiietrale Iwtwecn the cells of the pigment-layer. These

jmxresscs are llu' rods and fonen, and colWtively constitute the layer of rods and conea of (lit- iidult The bodies of the

KoMlsni pi. pi BtQ* 11 led I'plthcUiiTii iif Ihc eju I'luttr luniella uf the opUc enp,fl^ aceondary optic veiklcl ; r.rolinii(iniiprlain*llB of the optic cup); it, mKi^iUln nf (he optic cup. which farma the pun clllarls el Iriills rellnie: g, Til with blood-TCBKla : fv, tunlm tucuIou leiilla; U. blood-uurpuwlei : tA.ehon tf, lens-flben: It, leiu-eplthcllum ; r.nnu of the leni-tlbcr nuclei; ft, ftindaB at the iK>niea; he, exlernAl corneal epithelium.

rod- and cone-visual cells, situated on (he inner side of the i membrana limitan.s externa, are elongated into narrow ele- j ments, the position of tho nnctei being indicated by slight I enlai^ments. They constitnte the outer nuclear layer of ' the matnre retina. The outer nuclear layer and the layer of rods and cones are to be regarded, therefore, as one layer of highly specialized neuro-epithelium, made up of the rodvisual cells and the cone-visual cells, the inner segments or bodies of the cells being only apparently isolated from the outer segments, the rods and cones respectively, by the fact that the latter proj(?ct through minute apertures in the external limiting membrane. The axis-cylinder pnxjcssesof these cells pass toward the center of the eyeball.

The neuro-epithelium of the retina is the last of its elements to develop. In man and in many mammals, it is present at birth. In the cat and the rabbit, the rod- and cone-visual cells develop after birth, and hence the new-born of these species are blind. The macula lutea is developed after birth.

The cells of the inner part of the retina differentiate into the remaining nervous elements, some becoming the bipolar and other cells of the inner nuclear layer — the ganglion retinae — while others form the large ganglion cells of the ganglion-cell layer. The axis-cylinder processes of the ganglion cells are directed inward to form the nerve-flber layer, the fibers of this layer converging from all parts of the inner surface of the retina toward the optic disk or papilla. Here they perforate the retina, as well as the choroid and sclera, to pass, as optic nerve-fibers, to the brain.

This part of the optic cup, the ftmdus, produces then, in the manner descrilKjd above, the functionating portion of the retina, or the pars optica retinae, the anterior termination of which is indicated by the orra serrata.

The lenticular zone of the optic cup, which is in relation with the peripherj' of the lens, undergoes comparatively slight specialization. Its outer lamella is pigmented, as in the fundus of the cup. Its inner layer remains very thin and consists of cells which at first are cuboidal, but which later become cylindrical. At the end of the second month, or the beginning of the third, the two layers of the lenticular zone become plicated, owing to excessive growth in superficial extent. The folds are nearly parallel and are arranged radially with reference to the lens, the margin of which they surround. These folds are the first indication of the ciliary processes. The mcstKlermic tissue immediately external to th(» <)j)ti(! cup (lifforentiates into the uveal tract, the part corresponding with the lenticular zone of the cup furnishing the ciliary hody. The young growing connective tissue penetnitos between the folds of the lenticular zone of the cup, acupiiring intimate union with the pigment-layer, and thus provides the connective-tissue basis of the ciliary processes. This lenticular zone of the two hiyers of the optic cup, therci'on*, <'onstitutes the lining, or internal covering, of the ciliary ImmIv, and must necessarily be reganled as the continuation of the retina, it is known as the pars ciliaris retiiUB of the fully developed (?ye.

TIkj marginal zone of the optic cup, or the region bordering its orilice, is also related in its further growth with the uveal traet. Although in the earlier stages of development the lens lies in the mouth of the cuj), as time goes on the relation is so altered that the a[K»rture and the zone which borders it occupy a position in front of the lens. In this marginal zone both lamelhe of the cuj) become pigmented and aecjuire union with the layer oi' mes(Klermic tissue which is dillerentiatiiiir into the iris, and thev therefore contribute to the formation of that structure, ecmstituting its pigmentlayer. Th(^ pigment-layer of the ]>osterier surface of the iris is, therefore, an extended but rudimentary part of the retina. It is called the pars iridica retinae.

FnMu what has been said, it will be ap])arent that the retina f(>rms a (Mnnplete tunie with an anterior perforation, the pupil, and that it consists of the funetionally active part, or retina proper, th(» pars optica retinae; of the pars ciliaris retinae, marked olV iVom the latter by the ora s(»rrata ; and of the pars iridica retinae, which terminates at the margin of the pupil.

The evolution of the optic cuj) or secondary oj)tic vesicle mav b(» thus summarized :

I. Marginal or most anterior The thin atrof)liic pare iridica reregion of cup. tinir, or pi^rnicnt layer of the iris.

n. Ltnticular zone of cup. Pars ciliaris rt-tina*, covering inner

surface of ciliary body.

III. Fundus of cup. Functionating part of retina, or pars

optica retins, including : A. Outer layer. A. Pigment-layer of retina.

£. Inner layer. B. 1. Neuro-epithelial layer, made

up of layer of rods and cones (the processes of the rod- and cone-visual cells) ; membrana limitans externa; outer nuclear layer (the bodies of the rodand cone-cells).

2. Cerebral layer (representing an interpolated ganglion with connecting fibers), consisting of :

Outer reticular layer ; Inner nuclear layer ; Inner reticular layer; Ganglion-cell layer; Nerve-fiber laver.

The optic nerve is the metamorphosed stalk of the optic vesicle. When the distal and under surfaces of the vesicle suffer invagination, the stalk participates in the process, its under surface being marked by a groove which is a prolongation of the choroidal fissure of the optic cup (Fig. 163). By this infolding, the cavity of the stalk is obliterated and the stalk is converted into a double-walled tube enclosing mesodermic tissue which follows the invaginating ventral wall. In this mesodermic tissue is developed the arteria centralis retina. In mammals the invagination affects only the distal part of the stalk, the segment included between the eyeball and the point corresjionding in the adult to the place of entrance into the nerve of the central artery. It must be apparent that the outer layer of the tube thus formed is directly continuous with the outer layer of the optic cup, while the invaginated lamina is the prolongation of the inner wall of the cup or of the part that becomes the retina proper, since not only the distal wall of the optic, vesicle is invaginated, but its under or ventral wall as well.

the primitive optic nerve at this stage consists of layers of spindle-shaped cells, with a central core of vascular connective tissue.

The manner in which the nerve-fibers are developed is still a matter of controverjsy. According to His and K5lliker, the fibers gi*o\v out from the ganglion-cells of the optic thahimi and the anterior corpora quadrigemina^ while Muller and Froriep believe that they are the prolonged axis-cylinder processes of the ganglion-cells of the retina. According to Ramon y Cajal, growth occurs in both directions. In either ease, the cells of the optic stalk would furnish only the sustentative tissue of the nerve. There is also a contribution of sustentative tissue or stroma fnmi the mesoderm, as in the case of the central nervous svstem.

The Crystalline I^ens. — The lens, exclusive of its capsule, is, like the retina, of ectodermic origin. The first step in its <leveIopm(»nt is the formation of a thickened and deI)ressed patch of the ectoderm on the lateral surface of the hea<l, this area being situated at the place where the optic vesicle is nearest the surface (Fig. U)2, By d). The depression is the lens-pit. It soon becomes converted into a closed sac, the lens-vesicle, by the gradual a])proximation and union of its edges. The pit receding from the surface as its lips come together, the completed vesicle lies under the surface ectoderm, witli wliieh it is for a time connected by the slender stalk of tho invagination. Upon the disapi)earanc(» of the strand of cells constituting the stalk, the lensvesicle is completely isolated from the outer germ-layer (Kig. 1(12, (\i'l

The lens-voside in birds is a hollow epithelial sac several lavers thick, but in nuunmals the central cavitv contains a mass of (jells, which latter disappear in the later stages of development.

I^pon the invagination of th<» optic vesicle to form the se(M>ndary optic cu|>, the lens-vesich? is embracwl by the lips of the cup and still later Cannes to lie within the cup, near its orifice (Fig. 1^)4).

The further alterations in the vesicle are de|KMident primarily upon changes in its deep and sujH^rficial walls resjH^ctively, each of wliich consists of several layers of cylindrical cells. the cells of the sui>erficial wall alter their form, bcH?oming cul)oi<lal, while the posterior or decjwr cells lengthen so as to become fibers. Thus the deeper wall of the vesicle thickens at the expense of the central cavity — the central mass of cells at the same time disappearing — while the superficial layer remains thin. The two strata are continuous with each other at the equator of the lens, one form gradually merging into the other at this region, which is a zone of transition (Fig. 164).

The lens at this stage is composed, therefore, of a thin superficial or anterior stratum of cuboidal epithelial cells and a much thicker posterior or deep layer of so-called fibers, the latter being simply the greatly elongated cells of the posterior wall of the vesicle. Between the two laminse is a small remnant of the cavity of the vesicle. The epithelial layer persists throughout life as the epithelium of the lens, while the fibrous layer is the basis of the lens-fibers of the mature condition. The cavity sometimes persists as a small space containing a few drops of fluid, the liquor of Morgagni.

The next important stage in the development of the lens is the formation of additional lens-fibers. These result from the proliferation of the cells of the epithelial or anterior layer. The lens-fibers are formed in successive layers, as may be made evident by the maceration of a lens. Each fiber extends from the anterior to the posterior surface of the lens. The ends of the fibers meet each other along regular lines, producing thus the characteristic three-rayed figures or stars of the lens, one of which belongs to each surface. Hence, while the lens-fibers first formed are the elongated cells of the posterior layer of the lens-vesicle, the fibers of later gro^vth originate from the cells of the anterior wall. The epithelial character of the lens-fibers is evinced by the presence of a nucleus in each fiber of a young lens.

The lens-capsule results from the differentiation of the mesodermic tissue which surrounds the lens. It is from this enveloping vascular lamina, the tunica vasculosa lentis, that the growing lens derives its nutrition. The capsule is well marked in the second month. Its blood-vessels are derived from those of the vitreous body. At the end of the seventh month this well-developed, highly vascular membrane begins to undergo retrograde alterations, the final result of which is its transformation into the thin, non-vascular^ transparent capsii le of tlio mature lens.* The most active growth of the lens itself occurs prior to the degeneration of the tunica vasculosa lentis, so that even before the end of fetal life the lens has nearly attained its full size. Thus the weight of the lens of the new-born child is 123 milligrammes, while that of the adult lens is but 11)0 milligrammes (Huschke).

Hence the crystalline lens has a double origin, the lens-substance or lens proper being derived from the ectoderm, while the capsule oriirj nates from the mesoderm.

The Vitreous Body, — The vitreous body has been regarded usually as a roniparatively slightly differentiated form of connective tissue, and as being derived from the middle germ-layer. Recent investigtitions show, however, that it originates in ])art at least from ectodermal tissue. According to these observations, processes grow forth from those stromal elements of the optic cup which afterwanl Ix^come Midler's libers, and these processes, advancing toward the lens-vesiele, interlace to form a network, the primitive vitreous (Kolliker, Froriep). This ])roeess continues for a longer time at the marginal zone or month of the cup than elsewhere, the j>roto|)lasniie fil>ers which grow from this future ciliary and iridal j)ortion of the cup contributing to the lorniaiion of the zonule of Zinn. In mammals the cells of the lens-vesicle, another ectodermal structure, also send torth processes which, according to Lenhossek, bear a prominent part in the d<velo|)ment of the vitreons body. The mesodermic tissue, already in the sta<x<* <>f cnd)rvonal connective tissue, now gains access to the optic (Mip through the choroidal fissure (Fig. 1 <>•>), its ingrowth, in fact, accomjKinying the invagination of the un<l(M' >urta<M' of the opti(^ vesicle, and constitutes what K(")lliker designates the mesodermal vitreous. The intermingling of these two constituent ele ' It sonu'titiu's li:i|>|»«.Mis that parts of the fetal lons-capsulc persist. The most ootnmon exani]>le of 8\i<h persistence is the so-called meiiihrana pupillaris soinetinK*s present at hirth, pnuhicinK rmnjnn'tal otrtAia of the pupil. This results from the persistence (»f that part of the fetal capsule which is situated on the anterior surface of the lens, hehind the pupil.

THE MIDDLE AND OUTER TUNICS OF THE EYE. 339

ments produces finally the definitive vitreous. Since the inferior surface of the stalk of the vesicle — the future optic nerve — participates in the invagination of the optic cup, the mass of mesodermic tissue which helps to form the vitreous is continuous with that which invaginates the primitive optic nerve to produce the central artery of the retina. As a consequence, the blood-vessels which soon develop so plentifully in the vitreous b(xly are extensions from the central artery of the retina, the latter itself being continued forward as the hyaloid artery. The terminal branches of the hyaloid artery pass on through the vitreous body to terminate in the vascular capsule of the growing lens, constituting the blood-supply of that structure.

The intercellular substance of the young tissue undergoes but little differentiation, while the cells become gradually reduced to a few stellate elements which ultimately entirely disappear. The peripheral part of the tissue develops into the hyaloid membrane, which anteriorly acquires union with the capsule of the lens.

The blood-vessels of the vitreous disappear during the last two or three months of fetal life. The hyaloid artery persists, although in reduced form, for a longer time than the smaller vessels. Upon its final degeneration it is replaced by a canal, the hyaloid canal, or canal of Stilling, which is present in adult life.

The Middle or Vascular and the Outer or Fibrous Tunics of the Bye. — The outer fibrous coat of the eye, including the sclera and the cornea, and the middle tunic or uveal tract, comprising the choroid, the ciliary body, and the iris, are structures of mesodermic origin, being directly produced by the mesodermic tissue surrounding the optic cup. The richly cellular mesoderm applies itself to the exterior of the cup and differentiates into the two layers in question, the changes involving on the one hand the metamorphosis of the mesodermic cells chiefly into muscular and vascular elements, and on the other hand the evolution of a tissue essentially fibrous in structure. These two tunics are distinguishable in the sixth week.

the cornea is formed from the thin laver of mesoderm that penetrates l)et\veen the lens-vesicle and the surface ectoderm. The lens- vesicle lies very near the surface, and the thin stRitum of mesoderm that is interposed between the two is the anterior layer of the lens-capsule (Fig. 164). This anterior layer thick(»ns by the immigration of other cells and subsequently splits into two lamime, a superficial one which produces the cornea (Fig. 104, A), and a deeiK?r, which is now the proper anterior wall of the lens-c^apsule. Thus a space filled with fluid a pjwars between the primitive cornea and the lens, which (H)rresponds with the future anterior and posterior chambers of the eye, the <livisicm of the sj>ace into these two chambers being eflectcd subsequently by the development of the iris. The further (leveloi)meiit of the cornea consists simply in the differentiation of the mesodermic cells and the int(M'eellular substance into the several characteristic elements of the adult structure.

The uveal tract closely corresponds in extent with the two layers of the oi)tic cup. The choroid is differentiated from that portion of this primitive uveal tract which envelops the pars optica of the retina. In this region the enveloping layer of nicso<lcrniic cells develops into the several elements of the choroid, the most C()ns])icuous of which are an inner layer of capillary vessels, tlu^ choriocapillaris, and an outer lay(»r of larg(?r vessels, th(» stroma-layer of the choroid. the development of the clioroi<l bears a certain relation to the choroidal fissure of the optic cup. This tissure has been referred to as a gap in the iMi<lcr surface of the eup corresponding with the line of invagination through which the mesodermic tissue, of which the developing choroid is a part, grows into the cup to pnxluee the vitreous. Although normally this fissure in the retina entirely disjippears, its site be<*onies pigmented later than other regions of the pigment«laycr of the retina, and h<*nce there is, for a time, a clear streak in this part of the retina which has the appeanince of a fissure in that membrane. As the pigment-layer of the r(»tina was formerly assigncnl to the choroid, this streak appeared to be a breach of continuity of the ch<»roid ; hence the term choroidal fissure. In some cases, however, the choroidal fissure fails to close, and as the development of the choroid is largely dependent upon or is governed by that of the retina there remains a corresponding gap in the choroid. This defect enables the sclera to be seen from the interior in a line extending forward from the optic nerve entrance. It is known as coloboma of the choroid.

The ciliary body is developed immediately in advance of the choroid and from the same layer of mesodermic tissue. The deeper parts of the tissue in this region correspond with the plications of the ciliary part of the retina, sending processes into and between the radial folds of this part of the two layers of the optic cup, with which latter the highly vascular mesodermic tissue acquires firm union. This results in the formation of the ciliary processes. Some of the cells of the more peripheral part of this zone are converted into unstriated muscular tissue, thus producing the ciliary muscle. All the characteristic or important elements of the ciliary body are, therefore, derived from the mesoderm, while the thin layer of tissue on its inner surface, representing an undeveloped part of the optic cup, the pars ciliaris, is of ectodermic origin.

The iris, the most anterior zone of the uveal tunic, is produced from the same mesodermic tract that gives rise to the choroid and to the ciliary body. As stated above, soon after the lens-vesicle becomes constricted off from the surface ectoderm, it is enveloped by a mass of mesodermic cells which constitute its primitive capsule, and the layer of these cells lying between the lens-vesicle and the surface ectoderm splits into an anterior layer, which becomes the cornea, and a posterior stratum which is the anterior wall of the lens-capsule. This produces a space between the lens and the cornea. The lens now recedes farther from the surface, and the margins of the optic cup advance, so that the lens now lies within the cup, the marginal zone of the cup being in front of the lens, between it and the cornea, while its equator is in close relation with the ciliary regions of the cup and of the uveal tract. Thus the space between the lens and the cornea is

H divided into an iiiitcrior cdnipartnicnl, tlic anterior chamber,

H ntid a jmsterior fl])ai-p, tin- posterior chamber, the orifice of

■ tlic cup being a mi-nns of comiTHinication l>plnceii the twd

H and representing ttie pupil of a iaier sla^, the niar^ual

H zone of the cuj) furnishes the gniding line for the develo]* ^1 ment of the iris, The niesf«lcniiie tissue in rclutioD with

tm. Iffi.— BtgltEal iiectlon IhroUBh ttif pyp nf an embryo i

lUyi X 30 (Kailkerl: o,o|illc nttrc; j.. licmgimBl Hmni-nl-U

CllUry pun nf the rellnt ; p'. fi>rppiirl "f llie i>l.ll« PUp (rudiment of thB il

the srterli twnlralls retlnit unler II: f. trls; mj,, nienibr«n» pupllUriii «,«  with Piilib*tluro (,- pp.pn. iMiliwbne; I. lem; V. Icnt-cpllhcHum ; /. MlwottBjJ

the outer surface of the marginal stone of the euji difTerc tiatca into the vascular, muscular, and couneclive-tiasuc elM tnenta of the iris pn)iwr, while ils pfisterior pignient-laycr fa constituted by the slightly specialized layers of the mot anterior part of the optic cup, the part that in known a* th< para iridica retinre. Recent investigations (Xushhauni, Her^ zog, etc.) indicate that the circular and the radial muscular'J

fibers of the iris develop from the outer epithelial layer of the optic cup or, iu other words, from the part of the optic cup that becomes the pars iridica retina?. The circular fibers, sphincter pupillee, are distinguishable in the fourth month, the radial or dilator fibers, in the seventh mouth.

Since the anterior and posterior chambers of the eye are spaces hollowed out of the mesoderm, they represent a lymph-space and are, as such, lined with endothelial cells.

The cleft in the inferior wall of the optic cup referred to above as the choroidal fissure necessarily affects the marginal zone of the cup as well as the region posterior to it. If this part of the fissure persists, as it sometimes does, it may be accompanied by a corresponding deficiency in the tissues of the iris projjer. Such a congenital defect, appearing as a radial cleft in the lower half of the iris, is known as coloboma of the iris.

The Eyelids and the Lacrimal Apparatus. — The eyelids are developed from folds of the primitive epidermis that form over the superficial part of the developing eyeball (Fig. 165, pp and pa). After the separation of the lens- vesicle from the surface ectoderm, the latter pouches out into two little transverse folds for the upper and lower lids respectively. Each fold includes a certain quantity of mesodermic tissue, from which are produced the connectivetissue elem.ent^ of the lids, as the tarsal plates, etc. After the folds attain to a certain degree of development their eilges approach each other and become adherent, thus enclosing a space between the primitive lids and the front of the eyeball. The infolded ectodermic layers lining this space acquire the characteristic features of mucous membrane and constitute the epithelium of the coAJnnctiva, the part of this membrane that covers the cornea adhering closely to that structure as its anterior epithelial layer. The union of the edges of the lids begins in the third month and lasts until near the close of fetal life. A short time before birth the permanent palpebral fissure begins to form by the breaking down of the adhesions.

A part of the mesodermic tissue of the lids undergoes conversion into fibmiis connective tissue, thus producing the tarsal plates of the upper and lower lids^ with the iMJpebral DasciflB and tarsal ligaments by which the plates are attached to the margins of the orbit.

During the period when the edges of the lids are adherent, the Meibomian glands and the eye-lashes are formed. The ghmds develop from solid cords of epithelial cells that grow from the deepest or Malpighian layer of the primitive epidermis into the tarsal plates. The conls become hollow tubes by degeneration of their eentnd cells.

In addition to the two principal folds that produce the lids, a third, vertical fold app(\ars at the inner, nasal side of the conjunctival space, beneath the lids. This fold remains quit(i small in man and forms the plica semilunaris, but in most other vertebrates it attains much greater size as the third eyelid or nictitating membrane. A small part of this third fold develops sebacw)us glands and a few hair-follicles and becomes the lacrimal caruncle.

The lacrimal gland is devel(>]KHl in the same manner as tl)(i Meibomian glands, by the growth of solid epithelial cords from the conjunctiva. The cords grow into the underlying inesodcnn at th(» outer part of the line of reflection of the conjunctiva from the inner surface of the upj)er lid to the front of the eyeball. The conls acquire lateral branches and then become liollowcd out to form the secreting tubules and efferent ducts of the gland, the connective-tissue stroma of which is contribut<'d by the surrounding mesodermic tissue. The orifices of the adult efferent ducts in the upj>er outer j)art of the conjunctival sacj corr(»sjK)nd with the points from which the primitive cell-cords first grow forth.

Th(> efferent lacrimal apparatus, consisting of the nasal or lacrimal du<'t and the canaliculi, is related genetically to the growth of the nose and the upjxT jaw. S(M)n after the appearance of the nasofrontal ]>rocess, a lateral projection, the lateral nasal process, grows from its side near the base and advances <lownward so as to form the outer boundary of the nasal j)it and consequently of the future nostril (Fig. ()7, A, i>). This lateral nasal process is separated from the maxillary process of the first visceral arch by an oblique furrow, the naso-optic groove, which extends from the inner angle of the orbit to the outer side of the nostril, or, before the separation of the nasal pit from the primitive mouth, to the upper boundary of the latter orifice. The naso-optic groove indicates the situation of the lacrimal duct. By some authorities — Coste and Kolliker — it is believed that the duct results from the union of the edges of the groove. Later investigations seem to indicate, however, that the duct is formed by the hollowing out of a solid cord of epithelial cells that appears at the bottom of the furrow. In either case the epithelial lining of the duct is an ectodermic involution. When the nostrils are separated from the oral aperture by the union of the nasofrontal, the lateral nasal, and the maxillary processes (p. 133), the lower end of the furrow is obliterated, and the partially formed duct is made to terminate in the nasal cavity.

The canaliculi, representing the bifurcated upper extremity of the duct, result, according to one view, from the division of the upper end of the epithelial cord into two limbs, one for each lid, and their subsequent hollowing-out ; according to another, from the continuation of the cell-cord into the upper lid and the later addition of a limb for the canaliculus of the lower lid. The lacrimal sac is merely an expanded part of the duct.

THE DeVELOPMENT OF THE ORGAN OF HEARING.

As in the case of the other sense-organs, the auditory apparatus consists of highly specialized nenro-epiiheliiim, connected by nerve-fibers and interpolated ganglia with the central nervous system, and of protective and auxiliary structures. The neuro-epithelial structures, including the organ of Corti and the cells of the cristsB and maculae acusticae, result from the specialization of certain of the epithelial cells which line the membranous labyrinth. The perilymphatic space, which is a lymph-space, together with its bony walls, the osseous labyrinth, serve for the protection of the delicate neural elements, while the middle ear

ami tlio nxleriial oar act ; siinuroiis vibratiims.

Till' iiitcrniil ciir Iwiiig the css^scntial [lart of the organ of hciiriiif^ ami hriiij; alsd the part first formed may i»ro|wrIy n-ccive tirst eoiisiih-nit^m.

The Internal Car. — The membriuioiis labyrintli uf the

Uc vesicle of > ilors pit: B. Ihtf ol[c Malcle; rftirc Lrloderm,

iiilerMal ear is tlic <il<h'.-l part df the "i-jran i>f licanng. Its DiiM-iii is from a lliiekeiie.l .-in-iilar |»ateh i.|' eetiHlerm on the liorsulateral Mirl'ace i.l" the hea.l-rrjrimi of the eiiihryo near the liiirsil lerriiiitalioii i.f the fir.-l oilier viseerjil furrow. The tiiieketi.'.! arra M1ll^^ l>i-lo\v the Mirfar*', forniiiig thus the auditory pit, whic-li is iin>eiit in itie ihinl week (Kig, lOd, .1 ).

the j.il 1 nrs .h-e|.rr, it^ e.lut- a|.|.rua.-h eaeh other and

tiiially iiieel ami niiitc lo form the otic vesicle or otocyst. Tills "lilile e]>ithelial sae fira.lnally nve<les from the nirface eeli«l.'i-m. At llii,- Ma.L'i- "f d-veloimieiit then- is no eraiiial eapsule oiher than tlir imlilti'r<'nt itu-oileriaie tissue \v I lieli surrounds ihe hi-ain-ve.-ieles ; heuee, the otic vcsiele, cmbedded ill tlii,- ti»m-, lies in elo^,- proxhnily to tlw aftor!>n»iti, and e.mies into r<>Iation with the neiisli«)(aeial ^Mtiglion (jt. ;t"21). Till' vcsiele, at lirst s|ilierii-al, soon heeoiiies

THE lyTEEXAL BAH.

347

pear-shaped owing to the protrusion of its tlorsal wall. Tliis dursal projection, the recesaos vestibtiU or la.b3rrmthi (Pig. 16ii, C), lengthens out into a slender tube, the ductus endolymphaticns (Fig. ^^^), llie slightly dilated end of wlneli, the aaccua endolymphaticuB, is found in the adult occupying the aqucdiictus veslilndi of the tempoml bone. ,

Fig. 187.— Development or the membranniu labjrrlnth of tbo human ear (W. Hta, Jr.l : A. left labyrinlli of embryo of about four U'ecke, outer ildc; i>E, velUbutar and cochlear poitlons : rl. recessua Jabyrliithl. B, left labrrlnlh with part* nftoclalanaaudllfiry nerre* of embryo of about four ani a half weeka: rl. reoealus labyrintlii ; hc. ptc, ok, saperlar. poaterior. and external nemlelrciilar canalg ; I. Haccule: <. cochlea: vn./R. vdlbular and facial aervea; rg- rfi SB- veatlbular. cochlear, and genlciilatcganElin. C, left lahyrlnlh of embryo of about (Ivcwoclu, bom wllhnlit and below: labelllni; as In preredlngflKure.

The opiHJsite, anterior or ventral extremity of the otic vesicle tilsti bulges out into a small cvagiuatioD, wliieh gradually elongates until it is a tapering tube, slightly curved inward toward the median piano. This lengthens still more and becomes spirally coiled, forming the cochlear duct or scala media of the future cochlea (Fig. 168). The venicle it'^e If becomes constricted in such manner by an inward projection of its wall as to indicate its <livision into an upper larger and a lower smaller sac, the terms upper and lower referring respectively to the head-eud and the tail-end of the embryonic body. Before the con.strietion occurs, the wall of that part of the vesicle which is to become the future iipjKT or utricular division presents two {)ouched-out areas (Fig. I(j7, B). One of these gives rise to the extenud semicircular canal, while from the other are formed the saperior and posterior canals. The pouch that produces the external

Fig. 168.— niafrram to illustrate the ultimate condition of the membimnotu lAbjMnth (after Wnldeyer): i/, utrieulus: if, sacculus; cr, oanaliH rcuniens; r, ductus endolymphaticus: r. cochlea; k, blind sac of the cupola; r, vestibular blind cae of the du(?tu8 coehleuris.

canal is scmieirciilar in form and flat, lying in the horizontal plane, its upper and lower walls bring in contact with each other. The oppo.sed walls fuse, except at the periphery of the pocket, and hence all that remains of its cavity is a small marginal tube or channel, corresponding with its border and opening at each end into tlu* cuvity of the vesicle. Throughout the region of fusion of the walls, the latter become thin and finally disaj)i)oar, being replaced by connective tissue. Thus a semicircular epithelial tube is formed^ which is the horizontal or external semicircular canal. One end of the tube being dilated, the ampulla of the canal is produced.

the superior and posterior semicircular canals are formed in a somewhat similar inanucr by the other evaginatcd ponch or ])ockct, which is irregularly globular. To pro<bi<*e this result, the walls of the pocket contract adhesions throughout two regions, which (U)rresp<md with the rcs])cctive sj)accs enclosed by <'acli of the* two future canals in (jiiestion. The fusion of the walls takes place in such manner as to leave two narrow channels or tubes, one of which ahnost encircles the inner or mesial asjKHJt of the pocket, while the other bears the same relation to its jH)stcrior wall, the inner limb of the latter semicircle coinciding with the posterior limb of the former. The result of this arrangement is that two vertical semicircular canals are formed with their planes at right angles to each other, the two communicating with the otic vesicle by three openings, one of which is common to both canals. The other two apertures, being dilated, are the ampullated individiial orifices of the posterior and superior canals.

The constriction in the otic vesicle referred to above increases until this sac is divided into two parts, a larger, which includes the region from which the semicircular canals have developed and which is now the utricle, and a smaller vesicle, the sacculOi comprising the part from which the cochlear duct was evaginated (Fig. 168). The line of division coincides with the middle of the orifice of the ductus endolymphaticus, the proximal end of which participates in the division. Thus the ductus endolymphaticus becomes a Y-shaped tube, and affords the only bond of connection between the saccule and the utricle (Fig. 168).

The beginning of the cochlear duct, failing to keep pace in growth with the other parts, api)ears as a smaller tube relatively, and is known as the canalis reuniens (Fig. 168, cr).

The structures so far considered — the utricle, the saccule, the semicircular canals, and the cochlear duct — being the product of the ectodermic otic vesicle, represent simply the adult epithelial linings of those cavities. The fibrous layer of the membranous labyrinth, in common with the walls of the bony labyrinth, is a product of the enveloping mesodermic tissue. While the cells of the otic vesicle thus for the most part constitute the walls of the several sacs and canals of the primitive internal ear, some of the cells specialize into neuro-epithelium. The most marked specialization of this sort occurs in the cochlear duct, where most of the cells on that wall of the duct which may be called its floor — the part corresponding to the future membrana basilaris — undergo such profound modification in form as to produce the most highly specialized neuro-epitheliul cells anywhere to be found, the elements that constitute the organ of Corti.

In the utricle and the saccule, as well as in the ampulIsB of tlio semicircular C4inals, there is a similar but less marked sp(»cializati(m of epithelial cells to produce in the former case the maculae acusticse, and in the latter, the crista acusticfle of the ampnlhT. While, therefore, the cells of the otic vesicle which are to s(Tve as the lining mucous membrane of the membranous labyrinth become flattene<l polyhe<lral cells arranpMl as a sintrle lay(T, those cells which are to functionat(» as the periphenil part of th(» acoustic mechanism l>ecoine the specially modified C(>lumnar cells, many of them with cilium-like appendages, of the maculie, the cristae, and of the organ of C(>rti.

From the first the otic vesicle lies in close relation with the aeustieofacial ganglion (Fig. 167, />). As pointed out in a preceding chapter (p. 321), this ganglion subsequently divides into two parts, corresponding with the two divisions of the auditory nerve. This division (jf the ganglion and of th(* nerve is correlated with the separation of the otic vesicle into a coelilear part, the cochlear duct, and the two vestibular vesicles, the saccule and the utricle. While the cochlear duct IS still a short, slightly curved tube, the cochlear ])art of the ganglion lies in close proximity to the tube, in the concavity on its inner side. As the duct lengthens and becomes more coiled, tiie ganglion likewise lengthens into a band which follows the spiral course of the duct, lying parallel with the latter and on the side toward the axis about which it is coiled. Ai'ier the formation of the bony parts of the cochlea, this ganglion octMipies the sjnral canal of the modiolus and is known as the gangUon spirale. It helongs to the cochlear division of the auditorv nerve, which is distributed to the cochleii.

The remaining part of the acoustic ganglion becomes rather widely separated from the spiral ganglion, coining to occupy a position in tluMuternal auditorv meatus, and the part of the auditory nerve with which it is conne<*ted acquires relation with the macular regions of the utricle and saccule as well as with the crista' of the ampulhe of the semicircular canals. These nerve-fibers constitute the vestibular division of the auditory nerve, M'hile the ganglion is the vestibular ganglion or intumescentia ganglioformis of Scar])a.

The development of the bony labsrrinth of the internal ear^ as well as of the connective-tissue parts of the membranous labyrinth, is effected solely by the differentiation of the mesodermic tissue which surrounds the epithelial structures above considered. As previously stated, at the time when the otic vesicle is first formed there is no indication of a cranial capsule, the brain-vesicles being surrounded and separated from the ectoderm by indifferent mesodermic cells. During the progress of the alterations in the otic vesicle, this tissue undergoes condensation and alteration to form the membranous primordial cranium, and shortly thereafter the petrous portion of the temporal bone is outlined in cartilage by the further specialization of a portion of this primitive connective tissue. The formation of cartilage does not affect all of the tissue which is afterward represented by the petrosa, the region that borders the semicircular canals, the cochlear duct, the saccule, and the utricle remaining soft embryonal connective tissue. There is thus a cartilaginous ear-capsule produced which is more than large enough to contain the primitive epithelial labyrinth, and the walls of which are separated from the latter by embryonal connective tissue.

The bony semicircular canals are almost exact reproductions, on a larger scale, of the epithelial canals^ and they are formed by the ossification of the cartilaginous petrosa. Even before this ossification occurs the soft connective tissue between the cartilage and the epithelial semicircular canals differentiates into three layers. The inner layer, becoming more condensed, is converted into fibrous tissue, and, adhering to the epithelial walls of the canals, furnishes the connective-tissue component of the completed membranous canals. Its blood-vessels serve for the nutrition of the canals. The outer layer also undergoes condensation and forms a fibrovascular membrane, the perichondrium, which later becomes the internal periosteum of the bony canals. The middle layer, on the contrary, becomes softer — by the liquefaction nf tin* intercellular siul>stance ami the degeneration of the cells — so that gradually increasing, fluid-filled cavities make their ap|H*a ranee, and these latter becoming lar^(T and many of them coalescing, a ^pace is formed around the niemhranous canals which is filled with fluid, the perilymph. This perilymphatic space is bridged across at intervals by con nectivt»-t issue processes that serve for the convevanct* of l)hMMl-vess<*ls to the membranous canals.

The vestibule of the internal ear is formed in practically the same manner as the Imny semicircular canals, the epithelial saccule and utricle at'quirinj;^ their cimnective-t issue constituents in the same way. TheR* is the difference, however, that the bony v<»stibule dm^s not conform to the shape of the vestibular |mrts of the membranous labyrinth, since it is a sinjrle unilividinl cavitv enclosinjr the two little vesiclcs, the siiccule and the utricle.

The bony cochlea, while develo|K»d u|M)n the same general plan as the other parts of the bony labyrinth, presents certain cons|)iouous uKNlitications. The epithelial cochlear duct, as stati'd above, in its early sta^ is a short, tapering, and sli«rhtly curved tube. While it is still in this condition, chondritication of tlir petrous bone (K»curs, whereby the duct ac(|uircs its cartilaginous capsule (Ki^. 109, hk). This capsule is i)\\v\\ at the prnximal end of the duct and thronjrh this o|K*iiinLr lh<' cochlear bninches of the audittuy ncrvi' gain access U) the capsule, beinj; connected with the c(K'hlcar <livi>inn of the auditory «ran«rlion, which, owing to its prcviou>ly having a>>iuncd a jK>sitiou beside the duct, Ciuucs to be enclosed by th<* <'apsulc as the latter is formed (Kig. 1^>9, m\ <j^it). It is only after the chondrification that the ci>chlear duct lengthens out and becomes t>pirally coiled. The coiling is in such n)ann(>r that the cochlear nerve is surrounded bv the duct — thatis, it lies in the axis about which the duct is spindly wound. Within the cartilaginous capsidc, filling all the space not occupied by the spirally coiled duct and the ccH^hlear nerve with its lengthened-out ganglion, is the end)ryoni<* conn<»ctive tissue of which f(»nnerly the entin* cartilaginous {x^trosa ccmsisted.

Thu cochlea consists now of a spirally coiled epithelial tu!>e Ijiug within an eloiigati'd cavity in the cartilaginous petrosa, a cavity, the walls of which arc, therefore, cartilaginous. The peripheral wuU of the coiled tube is in contact with the inner surface of the wall of the iTnrtilaginous cnpfule (Fig. 169, x), a fact which has an imiHtrtaiit bearing upon ihc further stages of growth.

I

Tio, Ue,— Pan of ■ Bectloii Itiniugh the cocblea or an embryo Ml. II cm. (3.l> in.) long litter Boellchcrl: bt. cartl]*gln(nis capadte. In vhLch llie cocblmr duel describe* aseending »|"lral turns i ilf. duetUB t'orhletrin ; c. iirgun of Cortl; It, lamina vialibul«rt«: i, ouler wall of the membranous duclu«cochl«arl» nilh Ugamenlum aplrale; SV. acila vfitninll : ST*. S7'. acila tympanl; g. Kelallnoun tiaae, which itni Hlls the tcala Tvatlbulf inO In Ita I'M tniat: a\ remnant of the gelatinous tliiue, which I> not yet llquvllcd : M. Arm connective tluua summnillne the cochlear nerve lac); gip. ganglion splcnle; .v. nerrc which rum to Cortl'a or>n>n in (he nilure lamina aplrallanuea: r, c<im|<Hct cnnnL-dlre-tiune UfCT.wblch liepomo onined and shares In boundlBK ""o bone cochlear duct : P, pcrichon

The embryonal connective tissue within the capsule now undergoes important modifications, which vary greatly in different regions. That portion of this tissue which immediately envclo])ri the cochlear nerve becomes first dense connective tissue, which is afterward <lire<*tly coiiverte^l into bone, constituting the modiolus, or axis, of the cochlea. The processes of eondensation and subsequent ossification extend outward from the nuKliolus in a spiral line, which corresponds with the intervals bi^tween the successive turns of the cochlear duet, until they meet the wall of the original capsule, thus produein^ the bony cochlea. That is, by the development of this spiral plate and its connection internally with tlu» uKHliolus and externally with the wall of the capsule, a tub(* at first partly membranous and jwrtly cartilaginous, and at a later sta^e osseous, is produced, which encloses the mueli smaller cochlear duct, and like it is wound spirally arc>und the modiohis. To repcnit, the original sample eavity of the cartilaginous capsule is subdivided by the growth of the modiolus and of th(» spiral shelf in such manner as to become a t<jjinil/t/ coiUd tube.

The cochlear nerve, enclosed within the coil of the cochlear duct, semis branches (Fiji:. 1^^, -V) i»^ a continuous spiral line to th<* duct, and the soft tissue surrounding and supporting these branches condenses to form a connectiyetissue plate whicli extends outward from the modiolus tc the cochlear duct and which, therefore, has a spiral course about the modiobis, its entire inniM* edge being attached to that central axis, while; its outer border is, throughout its entire extent, in continuity with the inner wall of the duct. At a later sta^e tliis s])iral plate undergoes direct ossification to form tlic two lainclhe of the bony lamina spiralis. Thus it is that the ganglion s])ir:de and the successive terminal branches of the cochlear nerve come to be enclosed within th(; s[)iral lamina. Recalling the condition of the cochh»a before the growth of the spiral lamina, it will be seen that the latter, in connection with the epithelial cochlear duct, divides the tube into two parts (Fig. IfJO, aST, .ST). It will be evident, to(), that the epithelial cochlear duct now holds a relation to the* larger tube of the future bony cochlea which is similar in principh* to the relation of the membranous semicircular canals to th(» bony canals, but with the dilVcrcnce that the outer wall of the epithelial duct is in close contact with the outer wall of the future bonv canal at Xf and that the inner walls of the two are connected by a spiral plate, the lamina spiralis.

The cochlear duct, then, is surrounded by undifferentiated mesodermic tissue, except on the side farthest from the modiolus, where its wall is in contact with and finally adheres to the wall of the cartilaginous capsule. The lamina spiralis divides this tissue into two parts which respectively occupy the positions of the future scala vestibuli and scala tympani. Tliis soft embryonal tissue, as in the case of the corresponding tissue of the semicircular canals, develops differently in different regions. The innermost stratum, which is in relation with the epithelial cochlear duct, becomes fibrous connective tissue and constitutes the flbrons layer of the adult cochlear duct ; that is, on the side of the duct toward the scala tympani, it becomes the connective- tissue layer of the membrana basilaris, while on the side toward the scala vestibuli it forms the fibrous stratum of the membrane of Beissner (Fig. 169). The peripheral zone of indifferent tissue, that in contact with the now cartilaginous wall of the future bony cochlea, as well as that which lies against the lamina spiralis, also undergoes condensation and forms a fibrous, or fibrovascular, membrane, the internal perichondrinm or future periostenm. The tissue intervening between these two layers retrogrades, the cells degenerating and the intercellular substance liquefying, until finally the spaces known as the scala vestibuli and the scala tympani are hollowed out. These channels are lymph-spaces and the fluid they contain is the perilymph. This perilymphatic space is in communication with that of the vestibule. Therefore, while the cochlear duct or scala media encloses an epithelinm-lined space, as do the saccule, the utricle, and the membranous semicircular canals, and in common with those structures contains the so-called endolymph, the scala vestibuli and the scala tympani are in the same category with the perilymphatic spaces of the other parts of the internal ear.

The Middle and the External Ear.— The middle ear, consisting of the tympanic cavity and the Eustachian tube, is devclojx}il from the back part or dorsal ond of the first inner visceral fUrrow. The external ear, comprising the external auditory meatus and the auricle, comes from the dorsal extremity of the first outer furrow and the tissue about its margins, the tympanic membrane representing in part the closing membrane which sepanites the inner furrow from the outer.

The first inner viscend furrow, in common with the other inner furrows, is an evagination of the lateral wall of the primitive pharvngcal ciivity, or head-end of the guttract. The ventral end of this groove suffers obliteration, but the dorsal s(»gment, designate<l the tnbotympanic sulcus, becomes converted into a tube by the growing together of its edges. The tube is composed therefore of entodermic epithelial cells. It elongjites in the dorsal and outward dire(?tion, and its dorsal extremity becomes enlarged to produce the cavity of the tympanum, the remaining part of the canal becoming the epithelial lining of the Eustachian tabe. The canal being formed before the development of the cranium, and approximat(?ly its posterior half being surrounded bv the mesodermic embryonal connective tissue that al'terward becomes the petrosa of the temporal bone, the tympanic, cavity and a part of th(» Eustachian tube come to be enclosed within that bone, while the connective tissue enchasing the anterior part of the tube differentiates into the curved plate of ciirtilage that forms the cartilaginous part of the Eustachian tube.

Since the posterior end of the primitive epithelial tube insinuates itself between the otic vesicle and the surface, the tympanum comes to o(*cupy its normal position on the outer side of the internal ear. The tympanum, being derived from the back part of the first vis(»eral cleft, is in close relation with the first and second visceral arches, and the ossicles of the mid<ll(» ear ar(» derived from the dorsal extremities of the cartilaginous bars (jf these arches in the manner described in C-hapter XVIII. Necessarily the primitive ossicles are exterior to the primitive epithelial tympanic sac, as is also the chorda tympani nerve, which jKisses ulcDg its outer nide. After the ossification of the temporal hone, these structures are emhedded within the abundant soft connective tissue which is between the epithelial sac, now the mucous membrane, and the bony walls of the tympanum. This mass of soft tissue undergoes verj' considerable diminution, owing to which the mucous membrane comes into contact with the bony walls, and as a result the ossicles and the chorda tympani are enclosed in folds of the mucous membrane and seem to lie within the tympanic cavity,' They are excluded, however, from the true cavity of the tympjinum, since they are exterior to the epithelial or mucous- membrane layer.

Fin. 170. -Showing the gracjua] cluvi;lii|iiiii.-iil uf tlio fiartiinf the external car ftom promlncncss upon the mandibular and hyoldean Tistcral archus lHI»),varlnmly magnlHeil : 1. 2. prunilnenceB on mandibular arcb : 3. prominence between the two archea, prolooged postertorly in leconil fixture tu Sr; t.b. and 0, pniminencea on hyoidcau nr lecond riaceral arph; ^.lowerjaw. Prominence I forms the li^rua; 2, 3. ^, the helix; 4. Ihu anllbelli ; G, Ihc antitragug: S, the lubule.

The external auditory meatus is simply the persistent posterior part of the fir«t outer visceral furrow or hyomandibular cleft (sec pp. \\2, 1 Hil, this cleft doffing completely everywhere but in this region. The closing plate of the firrt cleft becomes the tympanic membrane. Hiiice the outer layer of this membrane is of wt.xlennie orijjin, while the inner layer is entodermic, being continuous with the epithelial tympanic lining, and the middle fibrous layer is derived from the mesoderm. The relation of the malleus to the membrane and of the latter to the bony tyinjKinie plate which forms part of the wall of the meatus is dealt with in the chapter on the development of the skeleton.

The auricle is ilerived from the tissue around the margin of the uucIoscmI hack part of the first outer cleft (Fig. 171, C). Six little elevations make their appearance here, the projections being mesodermic tissue covered with ectoderm. The mesodermic component of the elevations diiferentiates into the <':irtilaginous and other connective-tissue jKirts of the auricle. The nodules marked 2 and 3 in Fig. 170 bi^coming a continuous ridge, produce* the helix, while nodule 4 becomes the antihelix. The tragus and antitragus develop resijcctively from the projections 1 and 5. At the end of the second month, these parts are so far a<lvan(?ed as to be easily distinguishable, and the connective-tissue basis of the ridges and projections and the continuous plate-like mass to which they all are attache<l be^in to undergo chcmdrification. From the third month onward, this primitive auricle, by continued growth and greater sepanition from the si<le of the head, assumes more? and more the charactei>> of the fullv formed member. The lobule, however, which results from the growth of the little elevation marked G, lags behind the other parts in development and is rather indistinct until the fifth month, after which time it increases in size and gradually acquires its normal i)rop(>rtioiis.

THE DEVELOPMENT OF THE NOSE.

The nose is primarily a special sense-organ, although a pjirt of its cavity serves, in air-br(»athing vertebrates, as an adjunct to the respiratory system. The evolution of the mature organ of smell may be epitomized by the statement that the olfactory epithelium, the ess(>ntial part of this senseorgan, is a patch of dei)ressed or infolded ecjtoderm, the cells of which are highly specialized and ar(» brought into relation with the central nervous system by means of the outgrowth from the latter of a part of its mass, the olfactory lobe.

Verv wirly in intra-uterine life — before the end of the thinl week — the olfactory plates apjn^ar as loealizt^l thicken

rut:

iiigs of the ectoderm situated just in front of or above the on»l fossH. Tbese nasal areas are the forerunners of the future olfactory epithelium. It is worthy of note that the olfiictory [ilatf.* un.- \n very close relation with thf iirimiiry

— Devetupment or (he flu» nr ihc bum&neiDbryn (TUB) - A, embryo of it twenty-nluv dayi. The nuuCroalal plate dlOeren Hating into proccuui gioDularea, toward whlph the mailllnry proccsies of llret vlucral arcb are eitendiag. B. embryo of about Ihlrty-fourdajra: Ibe global ar, lateral frontal, and nuiillnry prufetwea are In apposition ; the prlmlllTe openlnt; la now better deflned, C, embryo of about the eighth week - Immediate Imiinilsrle* of moulh are more deflntte and the nasal oriHces are partly (brmed, external ear appearing. D. embryo at end of oecond munth.

fore-brain vesicle, being, in reality, on the outer surfiiee of the ectodermic covering of its ventral wall.

Owing to the rapid outgrowth of the surrounding tissue, the olfactory plates l)ecome relatively depressed, constituting now the nasal pits, which arc distinguishable at about the twenty-eighth day. The pits are separated from each other by a broad mass of tissue, the nasal or iia8ofh>ntal process (Fig.

171), which is, as it were, a localized thickening of the inesoderniic tissue on the ventral wall of the primary fore-brain vesicle ; and this process makes its appearance in the third week. During the fifth week the nasofrontal process thickens greatly along its lateral margins, the thick edges being known as the globular processes (Fig. 171, A, B). At the same time the lateral nasal processes bud out from the nasofrontal process, one on each side, above the nasal pits, and, growing downward, form the external boundaries of the pits, each of which depressions is bounded on its inner side by the corresponding globular process. The nasal pits, therefore, have well-marked walls on every side except below, where they are directly contimious with the oral fossa.

In the latter end of the sixth week the nasofrontal process, which, it will be remembered, constitutes the upper limit of the oral fossa, is joined on each side bv the united maxillary, and lateral nasal, processes. This effects a division between the oral fossa and the nasal pits, and forms, though as yet crudely, th(* external nose, and the upper lip as well. The detinite formation of the external nose may be said to be indicated about the cH/htli rack. The orifices of the na>al pit.-^ are now the anterior nares, while the pits themselves have bectnne short canals, opening by their deep orifices, the posterior nares, into the primitive mouth-cavity ahove the palatal shelves. The nanvs are separated from i'ach other by the still broad nasofrontal j)roeess. That portion of the nasofrontal process that separates the nares gradually becomes thinner and produces the septum of the nos(\ while its external or superticial })art gives rise to the bridge and tip of the organ.

The growth of the palate- shelves (Fig. 172) toward the median line, resulting in their union with each other and with the recently-fornn^d septum, definitely divides the nasal chambers from the cavity of the mouth, th(» posterior nares now opening into the pharynx. This separation is completed toward the end of the third month.

The complexity of the ndiilt nasal cavities js proclticetl W the formation of ridges ami jKHielies on (he lateral walls of the original nasal pits. Three inwardly projecting horizontal folds of the eeio<lernii(i lining of the cavity, tho superior, middle, and inferior tnrbi&al folds, appear njion the outer wall nf each nasal fossa (Fig. 173). Each fold contains a stratnni of mesodermie tissue which develops into cartilage and nubseqnenily into bone, forming respectively the three turbinated bones. The cartilaginons character of these folds becomes apparent at the end of the second, or the early part of the third, month. An cvagination on the lateral wall of each

Fin. ITS.— Roof or Ihe oral rnvlly ofa human emhirj-o with the niiidiunenta of (he poUlal prouenes (alter HIa), < 10.

nasal fossa, between the middle and the inferior tnrbinnl processes, becomes the antnun of Higbmore ; this is fnrmcil in the sixth month. Other cvafrinalions jirtxiiiec the ethmoidal, the frontal, and the sphenoidal sinuses, the last two of which are not completed, however, until after birth. Very early in the development of the nose a small invagination appears on the mesial wall of the nasal pit. In the fourth month of gestation this invagination has become n canal in the iipptum (Fig. 173, /), running from before backward and ending in a blind extremity. It is the so-called organ of Jacobson, which, in man, is merely a rudimentary strnctnre, but which, in most other mammals, is more highly developed, Iwing surrounded by a cartilaginons capsule and receiving a special nerve-supply from the olfactory nerve.

The olfactory plates lictKime sejiaratod from the fore-brain vesicle and ajii.-cijiiontly from the later brain and its oulgrowtli, the olfactwry bulb, by the development of au intervening bony plate, the cribriform lamina of the etlimoid bone. The ectoderraic cells of the olfactory plates differentiate into the highly specialized nenro-epithelial elements of the olfactory mncoiis monibranc, the olfactory epithelliun, and I their asiioeiuted supporting cells. The axon*; of the neuro- I epithelial cells piws upwuiil (liroiifrli die tribrlform ]»late of the ethmoid bone as the olfactory nerve-fibers, and, entering the ventral surface of the olfac'lory Ijnlb, arliorize with the proces-ses of the mitnd cells of ilie bull), whereby they acquire J functional relationship with the olfactory centers in the brain, 1

Fm. 173.— Cr SB a i i tl t tl tl hmd f ai en I rj i orown-rump meuureuient The iiuml cavlllw Bre (leen lot wlih thcoralisTilyal IhepUieii dealsniileiJ by a* K cBrUlHseaf tbeoual Mp-fl tuni;iH,turblnal»rlIl&ge J argan i f Jucnbson J the place when ItopaulBttfl Ihe naiial dbtUs' ; gf, paUul proceis; of. mftxlUary proceM; il. dBntal rld|».| |Hert»[g).

The esternal nose, as previously stated, first acquires defi- 1 nite form about the eighth week by the union of the distal J ends of the lateral nasal processes with the nasofrontal proo* 1 C8.S, the former proilucing the ala and the latter thebiid|«| and the tip of the nose. In the third month the organ is j tmduly flat and broad, but from this time on it gradually j a-ssume-s the familiar characteristic form. From the third i month to the fifth each external naris is closed by a gelat- J inous plug of epithelial cells.

+++++++++++++++++++++++++

CHAPTER XVII.

THE DEVELOPMENT OF THE MUSCULAR SYSTEM.

THE STRIATED OR VOLUNTARY MUSCLES.

The voluntary muscular system, genetically considered, is divisible into (1) the muscles of the trunk and (2) those of the extremities. The muscles of the trunk include two distinct sets : (a) the muscles of the trunk proper, or the skeletal muscles, and (b) the muscles of the visceral arches or the branchial muscles.

To arrive at a proper comprehension of the evolution of the muscular system it is necessary to revert to an important fundamental emhryological process, the segmentation of the body of the embryo, or, as it is sometimes expressed, the segmentation of the coelom, or body-cavity. As pointed out in Chapter IV., this process of segmentation occurs in all vertebrate animals and in some invertebrates.

The Muscles of the Trunk Proper.— At a very early stage of development the tracts of mesodermic tissue situated one on each side of the median longitudinal axis of the future embryonic body, the paraxial mesodermic tracts, undergo division or segmentation, in lines transverse to the long axis, into 'a series of pairs of irregularly cubical masses of mesodermic cells. These masses are the mesoblastic somites or primitive segments, often inappropriately called the protovertebrse. The somite first formed corresponds with the future occipital region, the second one lies immediately in front of the first, while two others, situated still more anteriorly, that is, near the cephalic end of the embryonic area, and seven more, behind the first, are added almost simultaneously. The formation of the primitive segments

tlicn pr(^>cociU tiiilwiinl until a considerable number have l)e(.'ii luldi'd. Tliorie in front of tlic one first formed are di'iiomiimted tlip head-segments, while the others are known iis the tnmk-segments. Kacli mniito ii> at first triangular in iToss-stt;fion, the haso of the triangle looking toward the chunla dor.sidi:*. Siibsc<iiiently they assume a more ciiboidal whiiiM'. In the lower vertebrates — amphibians and fishee — the somite is hollow, its cavity being in these cases a constricted-iift" portion of the IxKly-cavity (hence the term " 8^

el'iRciiuui tlMie iiiiting tnirt rb! roin wbo«e wall.

mentation of the eieloni" to exjin.-is this iirtHrsf). In the higher vert eh rates, liowevcr. the eavitv is obliterated by the eneroiielinieiit of the eells of the wall> of the soniit<-.

The eells of the somites soon iin(h'r<;o ditfereiitiation nnd rearm iifreii lent. It is nsiiiilly stated that, preparatory to the segmentation of the paraxial mesodermic tract, this tract has become separated from the remaining lateral plate of the mesoderm. The separation is not complete, however, and therefore, after the appearance of the primitive segments, each segment is connected with the more laterally placed lateral plate — by the separation of which latter into two lamellae the coelom is formed — by a smaller mass of tissue, the iieplirotome, also called the middle plate, or intermediate cell-mass (Fig. 17-!, vb). As development progresses the distinction between the primitive segment proper and the nephrotome becomes more sharply expressed, and the former is designated the myotome. The primitive segment on its mesial surface, near the point of union with the nephrotome, sends forth cells which form a mass called the sclerotome (Fig. 174, sk). The sclerotomes spread out and blend with each other, forming a continuous mass of tissue which envelops the chorda and the neural canal, and which also extends laterally between the myotomes, separating them from each other and constituting the ligamenta intermuscnlaria {vide p. 375) ; this tissue, being concerned in the production of the permanent vertebrse, has no further interest in this connection.

What remains of the primitive segment after the forma-* tion of the nephrotome and of the sclerotome is the myotome proper or the muscle-plate. Although, as previously stated, the primitive segments of the higher vertebrates contain no cavity, the myotome and the nephrotome each enclose a space, that belonging to the former being known as the myoccel. The myotomes or muscle-plates are so called because they give rise to the voluntary musculature of the trunk. But not all of the cells of the muscle-plate undergo transformation into muscular tissue. While the cells on the mesial or chordal side of the myoccel are going through certain alterations preparatory to their metamorphosis, the cells nearer the body-wall become rearranged to form a characteristic layer which is known as the cutis-plate from the fact that it contributes to the formation of the corium of the skin (Fig. 174, cp). The cutis-plate and the remaining part of the miisde-plate are conliniiniis around the myoctpl, the trunsition from one to the other being more or less gradMal. To suminarize, the primitive segment is differentiated into the nephrotome, the sclerotome, the myotome or mnaclaplate, and the cutis-plate.

The Metamorphosis of the Muscle-plate. — By the terra inujKle-ftlate. ia meant here the thickened layer of cells on the chordal or mesial side of the myotome proper, which layer condtitiites what remains of the myotome after the differentiation of the eutis-plate. These cells having pro- | liferated and increased in size, and having encroached ' thereby upon the cavity of the myotome, next undergo alteration in shape, becoming cylindrical, with their long axes parallel with that of the body of the embryo. The length of each cylindrical cell equals the thickness of the primitive segment, at least in the Amphibia and probably also | in the chick. The next step in the transformation is the ' acquisition of the transrerse Btriation characteristic of ver- j tebratc voluntary mupclc. Soon after this the protoplaf of tlic cell uudergoc];) longitudinal division iVito minute] fibrillie — which latter do not necessarily correspond, how- J ever, with the primitive fibrillfe of mature muscle — and tlw I cell-nucleus likewise divides. The metamorphosis of the f now tibrillated protoplasm into muscular tissue is first com— 1 pleted at the periphery of the fiber, so that a young musole- 1 fiber contains a central core of undifferentiated material, | including the daughter-nuclei resulting from the divis of the original nucleus. Soon after the appearance of stria- 1 tion and the fibrillation of the fil>er, the fibers begin to sepa^l rate from each other, and developing connective tissue witJtT young blood-vessels penetrates between them, the fibers now 1 showing aggregation into bundles. For some time longer i the fibers are naked, since the earcolemma is not acquired until considerably later. The differentiation into muscular . tissue gradually extends from the periphery of the fiber to 'J its core, the process being complete in the human embryo at J about the end of the fifth month for the muscles of the upper.jj extremities and in the seventh month for tho.-ie of the lower.

THE STRIATED OR VOLUNTARY MUSCLES. 367

The embryonic muscle-fibers are smaller than the mature elements and increase in size until the third month.

It is considered highly probable by most embryologists that muscle-fibers undergo multiplicatioii during embryonic life. There are several theories as to the method of this multiplication. The most generally accepted view is that put forth by Weismann, the essential feature of which is that the fibers multiply by longitudinal division or fission. Reference was made above to the repeated division of the nucleus of the cell as one of the initiatory steps in the formation of the muscle-fiber. According to the fission theory, there is one class of fibers in which the nuclei are arranged in a single row, and the fibers of this class do not undergo fission ; while there is another class, the fibers of which have their nuclei arranged in several rows. Fibers of the latter type divide longitudinally into as many daughter-fibers as there are rows of nuclei.

Although many of the details of the development of the muscular system are still involved in obscurity, it is a generally accepted fact that each fiber is derived from a single cell, the protoplasm of which develops the function of contractility to the subordination of the remaining vital properties of protoplasm. With this specialization of function there is nefcessarily a concomitant alteration of structure.

The muscular mass resulting from the transformation of each myotome grows in the ventral direction between the ectoderm and the parietal leaf of the mesoderm, or in other words into the somatopleure, to produce the muscular structures of the ventrolateral body-wall. The off-shoots of the myotomes which thus jKjnetrate the body-wall in the fourth week produce, in the fifth week, a muscle-mass which, for the most part, is non-segmental, and which gives rise to a dorsal and a ventrolateral division ; the dorsal division, derived from all the spinal myotomes, l)eing destined for the musculature of the back, while the ventrolateral division, springing from the thoracic myotomes alone, gives rise during the fifth, sixth, and seventh weeks to the muscles of the thoracic and abdominal walls (Banleen and Lewis *). The dorsal division extends in the dorsal direction, covering and acquiring points of attachment to the vert<}bral column, which has meanwhile Ix'cn forming. In addition to the ventnd and dorsal extension of the muscl(?-|)lates, each one grows both forward and backward — cephahid and caudad — in such manner that overhipping and intermingling result. During the diflTerentiation of the various muscular mass(\s from the myotomes, ventnd and dorsal l)ranches of the corresi)onding spinal nerves grow forth, their final distribution being to muscles developed from the particular myotcmies with which the respective nerv(»s correspond. According to Bardeen and Lewis the struc'turcs of the l)v)dv-wall are well differentiated by the end of the sixth week, although their extension to the mid-line is not completed until near the end of the third month.

What has been said above concerning the evolution of the trunk-musculature from the primitive s(»gments refers to those muscles that are develope<l from the segments of the trunk. As to the evolution of the head-segments comparatively little is definitely known. It is generally accepted thatin ela^niobranchs — a group including sharks and rays — there an? nine primitive segments in the region of the future head. The number present in mammalian embryos has not been clearly worked out. Three? oeeiptal and thirty-five spinal myotomes have been seen in human embryos of the fourth week, at which time the formation of myotomes is said to cease. In th(» l(>wer vertebrates each segment contains a eavitv lined with flattened e(»lls, the mesothelium, the metamorphosis of which into muscular tissue may be inferred to be essentially as alreadv outlined ai)ove. The first head-segment, which lies in contact with and partially envelops the optic vesicle, gives rise to the su|)erior rectus, the inferior rectus, antl the inferior t>bli(jue muscles of the eye-ball (innervated by the thinl cranial nerve) : the second segment produces the superior obli(jue (iiniervated by the fourth nerve); and the third, the external rectus (iiniervated by the sixth nerve). The fourth, fifth, and sixth segments al)ort and hence pnxluce

• Amerirnn Jtntrnal nj AniitomUj vol. L, No. 1.

no adult structures ; while the seventh, the eighth, and the ninth segments become metamorphosed into the muscles that connect the skull with the shoulder-girdle.

From recent studies^ it would appear that individual muscles undergo peculiar and significant migrations during their development, and that the origin of the nerve-supply of a muscle indicates the location of the particular myotome or myotomes from which it originated, since the segmental nerves are connected with their respective myotomes and supply the muscles derived from such myotomes. For example, the serratus magnus, being innervated by branches of the cervical nerves, develops from myotomes in the neck region, and subsequently moves down to become attached to the scapula and the ribs.

The Branchial Muscles. — This term embraces the muscles of mastication and the various muscles connected with the hyoid bone, with the jaws, and with the ossicles of the middle ear. They result from the metamorphosis of the mesothelimn of the visceral arches and acquire connections with structures that have arisen from the so-called mesenchymal cells of these arches or, in other words, from the embryonal connective tissue which makes up the chief part of their bulk. For an account of the growth of the visceral arches the reader is referred to Chapter VII. From this account and from that found in Chapter IV., it will be seen that the formation of the visceral arches and clefts is in reality the segmentation of the ventral mesoderm of the headregion of the embryo, or to express it in another way, it is the segmentation of the ventral coelom of that region. It is interesting to note that whereas in the trunk the segmentation of the mesoderm is restricted to the dorsal part of the body, in the head-region the ventral mesoderm also participates in the process. Hence the visceral arches, as might be exj)ected, consist of so many masses of mesodermic tissue, each arch containing a small («ivity lined with mesothelium, which cavity is a constricted-off part of the body-cavity or

• See '* Development of the Ventral Abdominal Walls in Man/* Franklin P. Mall, Johns Hopkins Papers, vol. iii., 1898.

Od'loiii. It is those mesotholial ct4l8 that produce, by their diift'rentiation, the niusoles under consideration. While so nuieh ooneerninjj: the origin of tliis group of muscles is practically assured bv ol)servations upon the embryos of the lower vertebrates, the details are still obscure. His assumes the origin of the palatoglossus, the styloglossus, and the levator palati from the second or hyoid arch ; of the stylopluayngeus, perhaps the palatopharsrngeus, the hyoglossus and the superior constrictor of the pharsrnx from the third arch ; and of the middle and inferior pharyngeal constrictors from the fourth arch. Further, it is held bv Rabl that the muscles of the i\u\\ including those of the scalp and the platysma — the muscles of expression — originate* from the mesothelium of the hyoid anrh in the form of a thin superficial sheet, which, gratlnally spreading out from the place of origin, breaks up into the intlividual muscles.

The Muscles of the Extremities. — The relation of i\\o (It'vclopnirnt ot'tlic inn>cles of tiie limbs to the myotomes i> >till a (li<j)uttMl point. Sdihc authorities hold that the linil)-mu-cK> of inaiiinials orii^inate from the mvotomes, as \\{\< >li()\vn l)v l)«)lirn U) l)c the ca>e with the fin-musculaturc of Selachian-. A tact adduced as a strong argument in t'avnr «>f tht'ir niyotomic oriiiiii is that the ncrve-su|)[)ly of each liiiil) ('(U'rc-jjonds with the nerves of the number of myotoniie x'unients in relr.tion with which the limb-bud <lcvelops (rid, |). loi)). ( )n th(* other iiand, it is stated* that the myotome- do n(»t extend into th(» developini:- limb-buds, but that the inn.-ch'< ol" the liinl).- are diil'ci-entiated from the mesenchvinal core ol' the liinh-bud, thi- procos following the entrance of the motor nerve-fihi'rs into the member. The mn>chs of the npper lind) are so well advanced in their devel<»|)ment by th(*' >ixth week a< to be individually distingni-hablc. th(»-^e (if the lower limb reaciiing a corresponding stau'e in the >eventh week.

^ r»anU*oii Mini Lc\vi>, /-*(•. cit.

THE INVOLUNTARY OR UNSTRIATED MUSCULAR TISSUE.

This variety of muscular tissue, like that considered above, is of mesodermic origin. But while the voluntary muscles arise from the flattened or mesothelial cells of the primitive segments, involuntary muscle results from the transformation of the embryonal connective-tissue elements, the mesencliymal cells, of the mesoderm. It is for this reason that some authors speak of the voluntary muscles as the mesothelial muscles and designate the involuntary muscular tissue as mesenchymal muscle.

While it is a generally accepted fact that each of the fibercells which make up nnstriated muscle is a metamorphosed mesenchymal or connective-tissue cell, the details of the process have not been accurately worked out. One may assume that necessarily the young connective-tissue cell elongates and that its protoplasm must undergo such differentiation as will fit it for the exercise of its future function, contractility.

THE CARDIAC MUSCLE.

The account of the development of the heart-muscle will be found in Chapter X.

+++++++++++++++++++++++++

CHAPTER XVIII. THE DEVELOPMENT OF THE SKELETON AND OF THE LIMBS.

Although the skeleton is the framework of the body in the aiiatoiuieal or meehanieal sense, it is not so embryologically, since its deveh)pnient is not l)egun, at least not to any important extent, until nearly all the prineijial organs are well differentiated, and its growth is largely subsidiary to that of the structures which, in the mature state, it supports and })r(>tects. M4»rph()logists s])eak of the exoskeleton and the endoskeleton, the former having reference to the hard structures found superficial to the soft parts, for whose protection they serve, such as the canipace of the lobster, and the hartl scales of certain fishes ; while the latter term si<rnifies the cartilairinous or bonv structures found within the bodies of most vertebrate animals. Kven in the highest vertebrates, certain bones, such as those (►f the vault of the cranium, are usually cousi(l(»r(»d by morphologists as l>eing thi' representatives <>f p;irt of the cx<>skeleton of lower ty])es.

The skeleton, using the W(>rd in its 4>rdinary sense, consists of the axial skeleton and the appendicular skeleton, or skeleton of the limbs, '^fhe former, inclu<liug the head and the trunk, is connnon to all vert(»brates ; the latter is not found in the lowest members of tliis class and hence is to \ye regarded as a later ac(|uisition in the evolution of the skeleton.

In studying the development of t]w skeleton, as in considering thatof otht'r systems an<l organs, cleaner conceptions of the y:rowth of the individual mav be obtained bv comparing it witli the evolution of the ty[)e. For example, the simplest form of skeletal apparatus is that of the amphioxns.

In this animal the only representative of the skeleton is the notochord, a cyliiulrical rod composed of cellular or gelatinous tissue in which neither cliondrification nor ossification ever takes place. Such an animal furnishes an example of the notochordal stage of the skeleton. the surrounding of the chorda with a sheath of embryonal connective tissue, by which it is strengthened and thereby better fitted to serve as the body-axis, furnishes the membranous type of skeleton, a stage a little farther advanced than the preceding. The next higher type of skeleton is the cartilaginous form. In tliiscase the eml)ryonal (ronnective tissue has undergone transformation into cartilage, at which p%int development is arrested, the stage of ossification never being attained. The cartilaginous type of skeleton is illustrated by that of the selachian (sharks and dog-fish).

The third and highest type of skeleton is the osseous. This results from the replacement of the cartilaginous tissue l)y bone. The process of ossification does not, however, affect every part of the cartilaginous skeleton, there being some portions of the latter which remain permanently unossified. As there are, throughout the vertebrate series of animals, various gra(hitions in the degree of differentiation of the skeleton, so in the course of development does the osseous system of every higher vertebrate pass througli these stages from the simplest condition, that of the notochordal skeleton, to the highest form of the almost completely ossified skeletal apparatus.

THE AXIAL SKELETON.

The axial skeleton, as stated above, includes the bones of the trunk and those of the head. Logically the development of the former will first claim attention.

The Development of the Trunk. The Stage of the Chorda.— The formation of the (jhorda dorsalis or notochord is the earliest indication of the axis of the embryonic body and it will be recalled that it is also one of the earliest embryological processes. The mode of development of the chorda from the entodermal epithelium has been described at p. 73. The chorda serves the purpose, as it were, of an axis about which the permanent vertebral column and a part of the skull are, at a much later date, built up. The anterior or headward termination of the chorda corresponds to the position of the later hypophyais, or pituitary body, and thus the chorda is coextensive, not only with the vertebral column, but also with a portion of the cranium. The cells of the chorda enlarge and become distended with fluid, the protoplasm of each cell being reduced to a thin layer. The peripheral cells, however, constituting a distinct layer, the chordal epithelinm, remain small, and it is by their proliferation that the •horda increases in size. In the amphioxus the chorda is the only " skeleton *' that is ever acquired, and in this animal it is a permanent structure. In all other vertebrates it becomes surrounded by embryonal connective tissue, mesenchyme, which latter undergoes chondrification, and in the higher types ossification also. While in some of the lower vertebrates, as in certain classes of fishes, the chonla persists as a structure of more or less importance, in the higher members of the series, birds and mammals, it retrogrades as the processes of chondrification and ossification go on, until finally it is represented only by the pulpy centers of the intervertebral disks.

The Membranous Stage. — The notochordal stage of

the development of the vertebral column is succeeded by the menihninous staire. The transformation is effected bv the appearance of an ensheathing mass composed of embryonal connective-tissue cells which surround not onlv the chorda but also the neural canal or fundament of the nervous svstem (Fig. 17-1, nh). The source of this embryonal connective tissue or mesenchyme bears an imi)ortant relation to the primitive segments. As the develo[)nient of the primitive segments was described in the last chapter, and also in Chapter IV., it will suffice to remind the reader that each primitive segment undergo(»s differentiation into the myotome or muscleplate, the cutis-plate, the nephrotome, and the sclerotome (Fig. 174), the sclerotouK? occupying the mesial surface of the segment and lying in close proximity to the chorda.

AVhile the myotome originates from the flattened or mesothelial cells of the primitive segment, the sclerotome is made up of cells of the type characteristic of young-growing connective tissue — that is, of the mesenchymal part of the primitive segments as distinguished from their mesothelium. Owing to the rapid multiplication of its cells, each sclerotome spreads out headward and caudalward, and dorsad and ventrad, surrounding both the chorda and the neural canal, until both these structures become enclosed in a common, continuous sheath of embryonal connective tissue. That part of this tissue which surrounds the chorda is often designated the skeletogenous sheath of the chorda and also the membranous primordial vertebral column. The cells of the sclerotomes not only surround the chorda and the neural canal, but they also spread out laterally into the intervals between the muscle-segments to constitute the ligamenta intermuscnlaria or the bands or strips of connective tissue which separate adjacent muscle

MuscU'Segments

Intersegmental arteries

ist spinal nerve

Ligamentum intermuscularium

2d spinal nerve

Ligamentum intermuscularium

jd spinal nerve

Skeletogenous sheath of chorda^ \^horda

Fig. 175.— Frontal projection fVom a series of sections through a cow embryo of

8.8 mm.(0.35 in.). (From Bonnet, after Froriep.)

segments from each other (Fig. 1 75). It is worthy of note that while this skeletogenous sheath of the chorda originates from segmented structures, the somites or primitive segments, and is to that extent related to the segmentation of the body, it now presents no trace of segmentation.

Very soon, however, this ensheathing membranous tissue exhibits areas of condensation alternating regularly with less dense areas. Eaeh such condensed area has the form of u somewhat obli(|uely i)lace<l bow or half-arch. This halfareli of condensed mesenchymal tissue is called the primitive vertebral bow by Froriep (^Figs. 175 and 176), whose investigations upon chick and cow embryos established most of the facts known concerning the development of the vertebra?.^ The median portion of the lx>w is on the ventral side of the cliorda and is known as the hypochordal brace. The lateral extremities of the bow abut against the eorresj>onding myotomes, eadi extremity becoming bifurcated. The dorsal limb of the bifurcation, the neural process, extends gradually over the dorsal surface of the primitive spinal cord, forming the neural arch; while the ventral limb advances ventrad, foreshadowing the hemal arch or costal process of the vertebra, or, as regards the thoracic region of the body, the future rib.' Both dorsal and ventral processes grow into the intervals between iidjaoeiit myotomes and hence are intersegmental, that i>, they, n> well as the vertebral \m)\\ from which they >j>rintr, (•orre>pon(l to the intervals between the primitive segments of the body. Sub>e(|iieiitly these j)roeesses of the bow uive rise to the variou«- processes of the ecmipleted vertebra. The median part ol' the bow, the hypochordal brace, subseqncMitly becomes cartilaginous and assists in forniiii<r th(» bodv of the vertebra in l)inl>, but in mammals it reuiaius unchondrified and becomes an inconspicuous and transitory part of the intervertebral ligament — the future intervertebral disk — exce]>t in the case of the first cervical vertebra, the atlas, the ventral arch of which it furnishes. The nieinbran(nis anlage of x\w cartilaginous body of the vertebra is fcnind in a special condensation of the mesen ' M<»re rcH'ently the ]>r(>cess lias K'cii stiKlicd m tlu* human embryo by IJiinh-eii. Afurrn'tin Jourunl nf Annfonv/. vol iv , No. 'J, \W\.

' Mnr]»h(>l()irir:illy, each vertehni is po^n^ssod of a ii'iirnl nrrh^ f(f>r tlio protection of the spnial cord ; and a bnnni nrrh^ for the protection of the ortnms of emulation, re<y)i I'll turn, and diirc^tion, the ril)> of man and the higher Vi'rtebnites U'lny the ])er»^istent hemal arches in tfie rei^ion of the tfiorax.

eliymatous slieath of tlio i-lionla on tin; caudal aide of the liyi>ocliortlal brace. The intarrertebral ligament is develowd fnmi the (erichonlal tissue on the dorsal side of the liyi>ocliordal bniee. Banleen's piimilivc rlluk iiiultides this aniiigc nf the iiitorvtTtebnil ligament phis the hyiwK'lionial brace of Froriep, which latter Bardceii regards as a transitory thicken of e n r 1 gi. Ilk

The Cartilaginous Stage.— This stage of the devcloi>niciit (if titc spine is bi-onght abont by the metamorphosis of IKirts of the niembrannus vertebral column into the cartilaKinons vertebra. Other and alternating \arts of the same strncttiro furnish the Intervertebral disks and the ligaments that hind together the individual elements of the spine. 'J'he hisliilogic:il changes neeessiry to effect the transformation of the embryonal connective tissue into cartilage are, briefly, the moving ajwrt of the cells and the modification of both the cells and the intercellular substance, the latter acquiring the characteristic qualities of the matrix of cartilage.

For each vertebral bodr there are two centers of chontlrifieation, one on each ^ide of the chorda within the mass of tissue referred to above (Fig, 176), The formation of cartilage l>egins in the second month. The two centers are soon connected with each other by a third, which lies on the ventral side of the chorda, the three forming now a cartilaginons half

crliuder wliicli js later coniplete<l by the development of c tilage on tLe dorsal side of the chorda (Fig. 177). Accord- ' ing to Bardeen, the tartilage of llie body grows at the exjwhse of the primitive disk anterior to (above) it. At the time when the chorda is completely encased in cartilage the Bi>inal cord is still ensheathed by merely membranous tig Before the end of the second month the neural uches of the J vertebrte are indicated by small isolated niassi's of cartila) which develop in the connective tissue BUrrounding the spinal I cord, the lateral parts of the membranous vertebral bows. 1

Parachordal carlilagt

In the eighth week these fuse with the bodies and appear I llieri as projections from them. By the end of the third I month the processes, or neural arches, have grown snfficiently ] to" meet with their fellows on the dorsal side of the spinal J conl, and in the fourth month the corresponding arches of , the two sides become niii ted, thuscompletingthecartilariiuma I sbeath of tbe cord.

The masses nf connective tissue occupying the intervalsj

between the vertebral bodies, originating, as stated above, in condensations of the mesenchymal sheath of the chorda on the dorsal aspect of the hypochordal braces, become the intervertebral ligaments (Bardeen's primitive disks) upon their fusion, in mammals, with the hypochordal braces. Subsequently they become the intervertebral disks. The tissue between the cartilaginous arches becomes differentiated into the ligamenta subflava.

While the unsegmented skeletogenous sheath of the chorda is gradually differentiating into the separate elements of the cartilaginous vertebral column, the chorda itself begins to retrograde. Within the bodies of the vertebrae its development is completely arrested, while those portions of it contained within the intervertebral disks continue to grow. The chorda at this stage consequently shows alternating enlargements and constrictions. In certain fishes it persists as a structure of more or less importance. In vertebrates above cartilaginous fishes, all traces of the parts of the chorda within the vertebral bodies are lost as soon as ossification occurs, while in the intervertebral disks parts of it jxjrsist as the soft pulpy cores of the latter.

Thus the cartilaginous vertebral bodies or centra originate in masses of mesenchyme situated between the primitive vertebral bows and are, according to Froriep, segmental, that is, they correspond in position with the muscle-segments, each centrum being developed within the limits of a single segment ; while the processes develop from the lateral parts of the vertebral bow and later unite with the body. Bardeen, on the other hand, refers the origin of each vertebral body to two segments, since, according to his observations, the body grows at the expense of the next anterior primitive disk.

The cartilaginous trunk is completed by the chondrifieation of the ligamenta intermuscularia to form the cartilaginous thorax.

The Osseous Stag^e. — The process of ossification begins in certain parts of the trunk at the end of the second month.

W\\\vv \\w work (»f olioiulrifiration is entirely completed. As i)ii« liiisioloirical details of Ixuio-foriiintion are to be found in I 111' h'\l-lMM)ks of lii>toloirv, it will not 1)C necessary to enter into till' suhjcct lu»ix'. The place's in any individual cartilage wlu-rr tissificMtion houiiis aix» called the cHintcrs of ossification. I'lh' proiM'ss is oni' of siii)stitutk>n, the cartilage becoming broki'ii down an<l absorlunl as the fonnati«)n of bone goes on. riit' oBHification of each vertebra is l)ogun at three conliT-n, onr in the bodv and one in cjieh arch. The centers l»»r the arches appear in the x'venth week. The centers ti>r ilic bodies a)){M'ar a little later and are found first in the d»n-;d MTtehiie, ap|)«'arinLr sueeessively later in the vei"t<»bi.i- I'iiiihrr lip and farther down. The ossified arches unite wtili liu'ji other diiriiiii: the lirst vear of life, but their nnicin Willi ihr ImmIv of the vertebra takes place betwtHjn the thinl .Old rii.ditli vciirs. At a much later periinl five accessory canU-.ia ul' H-.-i Ural ion are added to each vertebra. Two of these til I.. Oil to the body an<l jrive rise to two annular ]>lates of i.i.iu . I In- epiphyses, one for tlu' upper or cephalic surface and .•III lix ihr opposite t)r eautlal snrt:u*e. The remaining three I . (Oi I ■ briMiin rr>p<M*tively t<) the spinous process and the two ii.iii.»viii:ii» pi'orusses. The i*piphyses do not acrpi ire osseous Mil. Ill wiih ilu' vrrtrbra projuT until al)out the twenty-fifth

1 1 1. ... 1 .1 1 hi I transverse process of a e<'rvieal vertebra, enI III. I i.'i.oniii, and <'onsistini: of an aiiteriorand a j>osterior I III III liuh • iimrr than th«' tran>vcr>r process ])roper, since I. ml iiii iM \ i-iiir:i I port i(>iM> t hi' rudiment of a cervical rib. I '.II 1.1 ihi (Mitr (it' the fii>ion ot' this nidimentarv rib with I J, I. Ill ^.1 . pitMi-s, the vertebral artery, which passes i , . ,1 (lu III i' -iMTtimidctl by tlu» two processes, and thus .1, . h.ii I \ ii .d luiii-vcrM* processes di tier from those of ,1, I t, III III. f III the possession of a foramen.' I I. aUi- III. I ill*' rtxlH, bein^ strikintrly modified eervic^al

I I 111. 1 .1.1.1. li '.uiir iiiitlii>rili«'s. :iN Miiiot, that tht* >k)ium]()08 I ti.. ..«., I » . tiul iliiii thi' arti'iy j^rows through the* ossifying

I ii

vertebne, require special mention. The atlas contains less and the axis more than an ordinary vertebra, since that which corresponds to the body of the atlas never unites with it but fuses witli the body of the axis to constitute its odontoid process.

The atlas presents two centers of ossification for its neural arches — the so-called posterior arch — just as other vertebra? do. Unlike other vertebrae, these centers do not unite with the body but become joined to each other on the ventral side of the position of the cliorda by a piece of cartilage which results from the chondrification of the hypochordal brace, referred to on page 376. This forms the cartilaginous ventral or anterior arch of the atlas, which, in the first year of life, develops a center of ossification. The arch acquires bony union with the lateral parts between the fifth and sixth years.

The axis or epistropheus develops from the usual centers of ossification and from an additional one for its odontoid process. Bony union of the odontoid process with the proper bodv of the axis occurs in the seventh year. The odontoid process, in common with every other vertebral body, is traversed in the cartilaginous stage by the notochord.

The transverse processes of the lumbar vertebrse, like those in the cervical region, include not only the transverse process proi)er but also the rudiment of a rib.

The sacral vertebra each present the usual ossific centers. Inasmuch as they become articulated firmly with the pelvic bones and undergo fusion to form a single adult bone, the sacrum, their form is much modified during the course of development. The transverse processes of each side coalesce to form the lateral mass of the sacrum. Each transverse process consists, as in the cervical and the lumbar vertebrae, of the transverse process proper and a rudimentary rib, the center of ossification for the latter being quite distinct during early stages of development. The intervertebral disks of the sacral vertebrae begin to ossify in the eighteenth year, the process being completed in the twenty-fifth year.

Tlio coccygeal vertebrse are quite rudimentary. Each one is ossified from a single piece of cartilage, and usually from hut a single (H'uter of ossification. Occasionally the first piece of tli<.' coccyx <levelops from two ossific centers, the pnx?ess l)eginning at hiiili. Ossification begins in the second vert<*l)ra lu'twcen the lifth and the tenth years ; in the third, shortly l)elon' puberty ; in the fourth, scnm after puberty. The lower three pieces fuse into one before middle life, and this unites with the first, and the latter with the sacrum, at variable periods thereafter.

The Development of the Ribs and Stemtun. —

Ivcference has been made in the preceding pages to the liganieiita iulcriiuiscularia as strips or l)ands of embryonal conn<'('live tissue lying between adjacent muscle segments, which have ()rigiii;i(<'d, in conuunn with the sheath of the chorda, iVoiii tlx' c(>lls of the sclerotomes. TIh^ ligamenta intermnscularia become invaded by the costal j)roeesses of the primitiv<' v<'rt<'bral bows, th(^ costal process, which is the ventral division ol* the tip of tlw i)ow, growing ventnul and j)enetratiiig th(^ substance of the ligament to constitute a curved rod of connective tissue, the forerunner of the future rib. Thus (lu'rc are form<'d coiniective-tissne rej)resentatives of the ril)s, ench of which is enibedded in the looser connective tissue nl' th<' corresponding intei'nniscular ligiunent. It is b\ the development of* cartilage within these curved rods of conden-ed moenchvme, the membranous ribs, that the cartiliiginous ribs are jn'odnced. 'Vhv pi*ocess of chondrification commences in the >econd month, but does not involve the proximal <'n(U of the ribs, the tissue heie becoming ligamentous and servinir to bind to<reth<'r the ribs and the vertebi.e. Ivibs are formed throughout the entire extent of the Vertebral <M>lumn, except in the coccygeal region, but while in tlu' lower vertebrates the entire series goes on to mature de\.lt»|>meut, in mammals, including man, their growth is arrc-ted in the cervical, hunbar, and sacral regions. In the case ot' man and mammals only the thoracic ribs persist and iM'conh' adult structures.

As the distal (ventral) extremities of the ribs advance toward the ventral median line, the tips of the first five, six, or seven each exhibit an enlargement. These broadened ends soon coalesce, thus forming on either side of the median line a continuous strip of cartilage, the anlages of the sternum. The other ribs remain free at their ends. The sternum is therefore produced from two lateral halves, a circumstance that explains some of its anomalies, as for example, cleft sternum, which is a condition due to arrested development or deficiency of union.

The ossification of the ribs begins in the second month of fetal life and from a single center for each. The process does not involve the entire rib, a portion near the distal extremity remaining cartilaginous and becoming the adult costal cartilage. Accessory centers of ossification for the head and tubercle appear between the eighth and fourteenth years of life.

The ossification of the sternum proceeds from numerous centers. There is one for the manubrium and from six to tw^elve for the gladiolus. The ensiform acquires a center of ossification in the early years of life, but for the most part remains cartilaginous.

Although, as stated above, the ribs of adult human anatomy are limited to the thoracic region, their rudimentary representatives are found throughout the other regions of the vertebral column. In the cervical, lumbar, and vsacral regions each rudimentary rib becomes blended with the transverse proceas of the corresponding vertebra to form the transverse process of human anatomy. It is from the persistence of the seventh rudimentary cervical rib and its failure to fuse with the corresponding transverse process that the anomaly of a free cervipal rib results ; while the presence of a thirteenth or lumbar rib, as occasionally met with, is due to the unusual development of the first lumbar rudimentary rib.

The Development of the Head Skeleton.

Just as the wkck'tdii of tlif trunk consists of a doraally sihiatcd bony casir tor tlit- protection of the spinal con! aud a wrics of vcntrul nr licnisil arclics (or the protection of the orgiins of circiilutioii iiiiil respiration; so does the head skeleton comprise n Iiony eiise for the accommodation of the lirain with smaller ac(.^;ssory osseous coni[>artment3 for the orgiuis of special sense, as the orbits and the nasal chamlwrs; and also a ventrally situated ai>i>aratus which constitutex both a receptacle for the oral anil the pbarii'ngeal jxirts of the digestive system and a nicebanlsm for the mastication of

r.

<l..velop,.

rounding' th<> h<'adsimilar l.> that i.l" ■ till' ventral parts, -.v stni<'tTircs,<-oiistiliit from the nie.-ioilcnn

iirt. the cranial capsule, or brain-case, is exli'iit IVnm the c-i.iinective tis-^iic sai-id of tlic cliorila. its .>riL'in thus being r spiml chmm. On the other hand, he jaws and the liyojd bone and related ^' till- <(>-ealle<l viBceral skeleton, develop tissue of the visceial aicbes. As in the of ih.' trnrik skeleton, the eraniimi is first outlined in branous tis^n<' re>nliiii,n I'rom the dilliTcnliatioii of the embryonal connective tissue which ensheaths the head-enJ of the chorda, and also of the connective tissue of the visceral arches, this differentiation producing the membranotis primordial cranium. The metamorphosis of the membranous cranium into cartilage brings about the cartilaginous stage of the cranium, while the replacement of the cartilage by bone is the final step in the process.

Bones that develop from centers of ossification in previously formed masses of cartilage are styled primordial bones, while those that are pixxiuced independently of cartilage,, either in the skin covering the membranous cranium, or in the mucous membrane lining indentations in its walls, arc known as coveiing or dermal bones. The development of bone is therefore said to be either endochondral or membranous. For the most part, the bones of the base of the skull are of endochondral formation, while those of the vault are develoj)ed in membrane. The membranous or dermal bones are similar in point of origin to the exoskeleton — placoid and ganoid scales — of certain fishes.

The Membranous Cranium. — The membranous braincase is differentiated from the mesenchymal tissue which ensheaths the anterior or head-end of the chorda. As previously stated, the anterior end of the chonla is at a point ventrad to the mid-brain vesicle, in the angle formed by the latter with the fore-brain, at a position corresponding with that of the i)ituitary body (Fig. 178). The skeletogenous sheath of the chorda, in this situation as elsewhere, results from the multiplication of the cells of the sclerotomes, since this region of the body undergoes segmentation in common with the trunk. The number of bead-segments is uncertain. According to recent investigations upon shark embryos, there are at least nine primitive segments formed in the headregion.

The skeletogenous sheath of the chorda spreads out dorsad to cover the brain- vesicles. From the terminal point of the chorda, beneath the inter-brain, the sheath advances anteriorly to invest the fore-brain, which latter at this stage is bent over vcntrcul. From the part investing the fore-brain, a protiibcnint mass, the nasofrontal process, extends toward the j)riinitivo inoutli-cavity, constituting the anterior or upper h()iin<lary of the hitter. Meanwhile the mesencbsrmatic tissue of the visceral arches — that is, that part of the mesodcrnii(^ tissue of these structures wliich does not form muscular tissue — is un<lergoing similar transformation into menibninous tissue. The first visceral arch divides into an anterior or upp(T part, the maxillary process, and a posterior or hnvvv mass, the mandibular arch, these being the membninous jaw arches. The four jaw arches, with the nasofrontal process, form the boundaries of the primitive mouthcavity, the mandibular arches of the two sides having united in the median line to form its lower border, and the maxillary arches having fused with the lateral nasal and the nasofrontal j)n>cesses to c( institute its upper boundar}'.

"^rhe membranous primordial cranium, then, consists of a cnniplete connective-tissue investment for the brain-vesicles, of tlui nieiiibi-anous jaw arches, and of the hyoid and the branchial a relics, and presents in its walls the indications of the cavities for speeial-sens<» organs in the shape of the surface iuvauinations which constitute rtv-^pectivcly the otic vesicle, the Icns-vesieh', and the nasid pits.

The Cartilag^inous Cranium. — By the further differentiation of the memi)ranous cranium the cartilaginous stage is attainecl. The development of cartilage begins in the second month. \Vhih» thc^ membmnous cranium furnishes a coni]>let(* <*a])snle for the brain, tJK* cartilaginous brain-case is deficient, sinc(^ the process of ehondrification <loes not affect the i-egions of the future parietal and frontal bones. This is true at lea>t <>f man and the high<*r vertebrates. In those case^ where the >i<elet«)n rcMuains })ermanently cartilaginous, as in selachians (sliarks, <log-tish, ct<\), the entire brain-case partici])ates in the chondritying ])rocess. As the skull ext<'n(ls verv nnich farther forward than the end of the chorda — wliich latter terminat<*s at the j)osition of the future sella turcica — the regions of the |)rimitive skidl are designated respectively chordai and prechordal (Kolliker), or vertebral and everid>ral (Gegenbauer), according as they fall behind or in front of the end of the chorda.

The formation of cartilage begins in the region corresponding to the base of the future skull. On each side of the end of the chorda a mass or bar of cartilage is formed, extending forwand and backward, this pair of parallel bars being designated the parachordal GartUagflB(Fig. 179,1). Farther forward,

Fig. 1T9.— Kirtt tundameat of the cartilaginous primordial c Wlcderebelm) ; 1. J^ntStage! Cchordai jpE, parachordftl cartilage TV. Batbke'a trabecule cranll ; PR, passage for the bypophysla S, A, O naeal pit apcic veiiiclc, otncysl. 2. Second Stage: C, chorda; B, basilar plate TV trabeculie cranll, which have become united In front to eonrtilute the nagal Beptum (S) and the ethmoid plat«; a.AF, proccsBusnf the ethmoid plate enclosing the naial otsan ; (H, foramina olfactorla for (he passage of the olfactory ni^nca FF poilorhltal process; SK. nasal pit; A, 0. optic and lahyrtnihlne veskle*

in the prechordal r^Ion, another pair of cartilaginous masnea is producetl, known as the trabetmla cituiU. The latter are not straight bars, but have somewhat the form of a pair of calipers. In a short time the cranial tni)>eculfe unite with each other, but not throughout their entire extent, an aperture being left at the position of the pituitary body. It is through this aperture that the oropharyngeal diverticuluni, which forms the anterior lobe of the pituitary body, projects to come into rolatioa %vith the diverticulum from the inter-brain, which pnxliices the posterior lol)e. At a later period ossification fM-curs lien*, as elsewhere in the Ikiso of the skull, thus eoniph'tely isolating the pituitary IkkIv from the wall of the j)harynx. The jKiraehonhil eartilages also fuse with each other an<l with the eraniti! tral)ecula?, the four pieces now foniiinji: <»»H^ mass. The j»n)oess of ehomlrification extends to iither parts of the memhranous enuiiuni so as to produce a eartilagiiKnis hrain-ease, just as, in the case of the vertebral column, the «lorsal extrusion of cartilage-formation gives rise to a case (»r canal for the sj»inal cord. As before stated, however, the chondrifyiug j)nx'ess does not affect the entire niembnuious cranium in the higher vertebnites, chondrificatiou oceurring around the ]»osition of the foramen magnum and in the lateral walls of the cranial capside, while parts of tli(? vault remain membranous. The anterior extremities of \\n\ unite(l cranial trahecuhe become so modified in form as to constitute the plate of the ethmoid and the nasal capsule for the lo<lg('ment of th<» olfactory epithelium. In each lateral n'<rion the cartilaginous ear capsule is differentiated.

Meanwhile the cartilaginous visceral skeleton is developing from the memhranous .-truetures of the visceral arches. As in the ease of the hniin-eapsule, the ehondrifying process does not involve all \yav{< of the membranous visceral skeleton, ]>arts of the latter heing replaced later by dermal or c<^vering l)one — that i<, bones that develop in membrane without having been ])revion>ly mapped out in cartilage.

In tlu; first visceral arch, the formation of cartilage occurs only in the mandil)nlar jMU'tion, the maxillary pnx'ess contiimintr memhranou*^. The eartilairc <>f the mandibular arch a])p<'ars in the form of a eurved bar running ventrodorsally. '^rhi> bar divides into a smaller j)roximal or dorstd piece, the palato(juadratum of comparative luiatomy, and a longer distal or ventral s(^gment, Meckel's cartilage, "^rhe ])alato-quadratum sul)s<'(|nently divides into two ])arts, the cartilaginous anlages resj)ectiv<'ly ol* tin- ])aIato-j)teryg<)id plate and the incus. MeckeFs cartilage like\vi>e undergoes division, there being se])arate<l from the chief mass a small ju'oximal st»gment called the arti<*ulare, which is the forerunner of the future malleus. Thus th(» cartilaginous bar of the mandibular arch has to do with the formation of certain of the ossicles of the middle ear as well as, to a limited extent, with the development of the mandible.

In the second visceral or anterior hyoid arch, chondrification also occurs, but not throughout its entire extent. A bar of cartilage, the hyoid bar or Beichert's cartilage, is produced in this arch and undergoes division into three segments, of which the proximal or dorsal is the forerunner of the future stapes of the middle ear, while the other two pieces represent respectively the styloid process and the lesser horn of the hyoid bone. The tissue intervening between the position of the styloid process and the lesser hyoid cornu does not chondrify in man but remains membranous and becomes the stylohyoid ligament (see Fig. 185).

In the third visceral arch, or the posterior hyoid arch, a rod of cartilage develops which represents the greater cornu of the future hyoid bone. Ventral to this, there is formed a median unpaired piece of cartilage, the copula, belonging to the arches of the two sides, which later develops into the body of the os hyoides.

To summarize, the head skeleton in the cartilaginous stage of development presents an imperfect cartilaginous brain-case, capsules for the organs of smell, sight, and hearing, and a cartilaginous visceral skeleton, the several parts of which map out the lower jaw, the hyoid bone, the styloid process, and the ossicles of the middle ear.

The Osseous Stage. — The bony condition of the head skeleton is brought about in part by the development of bone from centers of ossification in the cartilages described above, and in part by the growth of covering or dermal bones in the integument covering those areas which are deficient in cartilage ; in other words, by both endochondral and membranous ossification. It may be stated in general terms that the bones of the baseand of the sidesof the skull, including the auditory ossicles, the ethmoid, and the inferior turbinated bone, are produced by ossification in cartilage and are hence called primoi'diaJ bones; and that the bones of the vault of the cranium, and for the most part of the face, result from the membranous method of

nssi Rent ion, unci :irp tlicrcforo stylod (Icrmal or covering bones. Smio of tii(! iiulivkliial bones, however, are partly of carlila}rinouij and partly of nicnibraiioiia origin, the several {Mrtioiis reniuining iwrnmncntly distinct in certain lower vertebrates, but in niEin nuiting so iutiniately witli (iich otlitT a» to present no trai-e tif their previously separate comlition.

The occipital bone consists of

two genetically distinct jKirts,

the )iUiK>rior or intetpatietal -por tion, which \n a dermal bone,

and the occipital bone proper,

rijiin. The OBsiflcation of the latter

(Hie on each side of the fommen

mitions, one in frtmt of the foramen

ihi' liii^ilar process, and one (losterior to that fli)Ort«re for

III' lalmliir imnioii nfilic !»iiie ni>t belonging to the iuter >ial Minium. ( Wiliciiiinn lnjiiii^ in these centers early

K' ihird li'ial ni«n[h and pr«<-<rds at ^yv]\ rate that at tlic

ol' bii'ih ihi- lioin' n>n-^i-.ts of fmir bony jwns which are 

tliii) hiyers of cartilage.

tiiaiii separate tlntingboiit

ijrir-ls, iTsjteetively, the ex supra-occipital (Fig. 181).

ihennion with it of the in riial bone that ossifies from

! wifli the .-iUpra-oiK^iintal

iilli ot'fi'tal life. Consisting at panilcil liv caililagc, the oceip 1 be<-onii-s a ,-ii,._de l...nc by the end i.f the thin! or fourth

ir by thi- buiiy uiiiuii c.fllie sciKinitt' se-rmeiits.'

J'lic temporal bone is made up of thi-ee genetically distinct

' In s.iiiir- m^,-' \}\i- union of llir iii(i'rii:irifl;il willi llii' sii]irH-<>n-ipitnl is iiniili'iiv Ihi' ailiill Ihiiu' tluii iiiTwiitin;! Iwii iniusvcrau linsures which s, iiui: rriilii LMi'li laliTjl :iii^lv, tiiwunl tlio lULiliun line.

parts, the smuunoBal or BqaamosTgomatic, the petrosal or petrom&stoid or periotic, and the tympanic. At the time of birth these three elements of the bone are still separate from eacli other, the tympanic being an incomplete ring, and the petro

mastoid being still without a mastoid process. The petromastoid is the only part of the temporal bone that is outlined in cartilage, the squamozygomatic and the tympanic being represented in the eartilaginous stage of the cranium by mem bran OU3 tissue.

The. sqnunosygomatic (Fig. 182) is ossified in previously

Fig. l^J.— S«iuamozy«<>mjitic 1^7) and tyin|»aiiic ('), of t«'iu|M>ral IxtUL' at birth.

formed niemhrano from a single center of ossification, which ai>j)oar.s in the lower part of this segment at about the seventh \v<»('k. The proeess of bone-formation extends in all directions from this center, but especially upward into the squamosa and outward and forwaril into the zygoma. The periotic or petromastoid results from the ossification of the cartilaginous ear-oaj>sule, which latter constitutes a part of the cartilaginous portion of the early cranium. It should be remembered that the essential part of the orgiui of hearing, the internal ear, is differentiated from a small pouch of epithelium, the otic vesicle, wliieh is produced by an infolding or iiivairination of the surface* ectoderm, and that it is the cartilat^inniis tissue cuclosiuir the otic vesicle and its outgrowths, the semicircular canals and the cochlea, that constitutes the cartilai^iuous car-capsulc.

The (Ksitication of (he ])criotic is usually descrilx?d as pro(•('(Mlinir iVom three ceuter<. The first of these, the opisthotic, inake< its apix-araiiec iu the hitter part of the fifth month on the outer wall ot' th<' ea]>sule, at a poiut corresj)onding to the ])o>i{iou of the |U"ouiout<»ry, \\ heuee the formati<m of bone -preads iu such uiauuer a> to ]>ro(lu(M' that part of the petros:i which is below the iuterual auditory eanal. A secoml center, the pro-otic, aj>]>eais a little later over the superior seniiein'ular eaual aud <::ive< rise to that ])art of the ])etrosa above the iutiMMial auditory uieatus, aud also to the inner and up|)er part of the uia>toidea. The third nucleus, the epiotic, arises iu the ueiiihborhood «»f the ])o-iterior semicircular eanal. ( )>sifieatiou proeee(N rapidly, the three parts speedily uniting to t'orui oue boue, the [xriotic or petroiuastoi<l. The |)etrous portion of the ju'riotic is the ruon' important and the more constant. I'he luastoid is of variable size in different animals, and in the human sp<'cies, at birth, it is fiat and devoid of the ma>toi<l pnx'ess w hich is so conspicuous in the mature condition of the skull. The mastoid process develops during the first two years of life, but its air-cells do not appear until near the age of puberty.

The pars tyznpamctis, or the tsrmpanic (Fig. 182), whicli constitutes the bony part of the wall of the external auditory meatus, is ossified in membrane from a single center of ossification. This center appears in the third fetal month in the lower part of the membranous wall of the external canal, from which point the pnxiess of bone-formation extends upward on either side so as to form an incomplete bony ring, open above. This tympanic ring is situated external to both the ear capsule and the ossicles of the middle ear and gives attachment to the periphery of the tympanic membrane. The further growth of the tympanic ring being in the outwanl direction, it becomes a curved plate or imperfect cylinder of bone which constitutes the bony wall of the external auditory canal. At birth, the pars tympanicus still has the form of the incomplete ring, its further development taking place during the first few years of life. The extremities of the ring unite with the squamozygomatic before birth. The tympanic unites also with the petrosa except in a region adjacent to the proximal end of Meckel's cartilage, where an aperture is left which is the petrotympanic or Glaserian fissure. Since upon the part of Meckel's cartilage which is thus enclosed bv the union of the two bones is formed the long process of the malleus, the presence of this process in the Glaserian fissure is accounted for.

The stybid process of the temporal bone belongs to the visceral-arch skeleton. It ossifies in two parts in small masses of cartilage that belong to the anterior hyoid arch. One, the tympanohyal, gives rise to the base of the process (Fig. 186); it begins to ossify before birth and soon unites with the temporal. The other segment, the stylohyal, undergoes ossification later and joins with the tympanohyal only after adult age is reached. Sometimes it remains separate throughout life.

The sphenoid bone is for the most part ossified in cartilage. The body of the bone is represented in the fetus by two sej>arate parts, the posterior body, or basisphenoid, or post

.spli<'noi<l (Fijr. 1S.3, hs\, wliicli incliulos all that part of the IhmIv ni* th<» niatiiro bono whicli is jx)stcrior to the olivary rinlnrnco and to whirh belong the greater wings (alisphenoitls): and an anterior body or presphenoid (;>^), situattnl in front of the olivary ominonoe, to which belong th|? lesser wings (orbitos|)h('noids). The ossification of the basisphenoid {)roeeeds from two centers placed side by side, which

%^

/^

Ki«.. l.s:;. - Splu'noiil Imhu\ fiflh or sixth A tnl month: seen from above: p«. pre>I))h-iiui«1 itr iiiiti-riur Ixuiy, \\ itti h'sscr wiii}:-^; (i{(, greater wings ; 6<, baffisphenoid

or |»i»'ti'i lur I'luly.

i|»jMar in the eii

:htli w(M'k. 1\vo months later two secniid.-irv (M-nh rs M])pear for the lateral parts of the b(Kly.

'rih- presphenoid likewise develops fnjni two centers, which nre appMniit in ilie ninth week. The union of the |)ir-plu'n<)id with tlir basisplieiioid occurs in the seventh or eiL'lith nionili. Maeli greater wing develops from a sinprle eenlci* of o»ili<:ition, uliieli is ]>resent in the eighth week. The pmrrssof o>sifieaiion >j)read> from this center to produce imi (nilv the LTi'eatrr wini: but also the external pterygoid j)lal('. i'lie ureatrr win^s remain separate from the body until <nnie tiinedurinsr tli(» lir>t vear after birth. P^aeh lesser wing <»s>ities from a center that appears about the ninth wrcL. 11ie les<er >vinij:s unite with the j)rcsj)henoid in the >i\ih fetal montii.

The internal pterygoid plate dilfers from the other parts of the .vpheiioiify in cartilage but in membrane. It is stated. howev(»r, that its hamular process tir-^t hreomes eartilaiLrinons bet'ore it ossities. It is, therefore, a run rin(/ hum'. \\< center or <'enters of os>itication aj>pear in the fourth month in the connective tissue in the lateral walls of the oro])harvnireal t-avitv. In manv animals this plat<' ac(|uires n«» connecti(»n with the external pterygoid plate, but remains throughout life a distinct l>one, the ptervgoiil. In man it fuses witli the external plate in the fifth month.

The presphenoid with its attaclied leaser wings, and the basisphenoid, to which are united the greater wings and the pterygoid plates, remain permanently separate bones in some animals. In man, as noted above, the two parts of the body of the bone unite shortly before birth, although the greater wings remain separate until some ranutha after that event.

The etlunoid bone and the Inferior turbinate are formed in cartilage, resulting from the ossification of the jKiHterior portion of the cartilaginous nasal capsule (Fig. 184, m). The

vtlli Ih^ nntl ruTltj at (hcplMea dmtEnntri by n 't K. »rt1lnge»t ibe iiikwI Hptum ; n, (urbliml cartilage ; J. omnn nf Jacobsnn : J', the iilaoe wbeie II apclW into tlia nasal raviij- ; gf, palatal proeeia; of, maiillarj pruccu; iJ. dental rtdgc

IHuTlWlB).

latter represents the anterior extension of the cartilaginous trabecule cranii so mmlified as to constitute a rc<»ptacle for the olfactory epithelium. The anterior part of this capsule remains cartilaginous throughout life as the septal and lateral cartilages of the nose. By the ossification of the posterior part of the nasal capsule the ethmoid and the inferior turbinate bones are produced. Ossification, beginning in the fifth month, involves the lower and the middle turbinals and a psirt of the lateral masses. The ixwificatlon of the superior turbinal, of the vertical plate, of the crista galli, and of the

rcintiiiiinjL^ jxirts <»f tho lateral masse? is efiected after birth. Tlic Imiiiv iiiiinii <»f till' lateral masses with the median plate i> <'oinplct(*<l U'twii'ii the fifth ami seventh years.

Tlif frontal bone is a ot evening or dermal l)one, being ossitird ill iiK'inhran*.' tVom two centers of ossificatioUy one for earh iatt-nil half. These centers are situateil above the <)rl>ital arrhrs and are tir>t a]»|mrent in the seventh week. At hirtli, tin* two halves of the bone are still se|)aratey their niiiuii not nr<Mirriiig until during the first year of life. Sonieiinic> till* union fails to take platv, the ciimlitiou of the per>i>t<Mit frontal or metopic suture being known lus metopism. M<toj»i-in is ronsid(*ral)ly more common in European skulls than in tlm.-t' «»f lower tyjH'.

TIk- parietal bone i< also o.-sificnl in membrane. It develops from tuo mhlci wliirli soon c<»alesce. Their position eonx?-jMnid- to that of tile future parietal eminence.

TIh.' bones of the face are for the most jxirt dermal l>ones. < )f tin-**, tin* nji|K'r and the lower maxilhe and the palate Imiiu- IxJoiiLT to the vi-ccral-arch skeleton. The others devrlnp ill tin- iiHinhranous wall <>f the cranial eapside.

Tlw nasal jithI lacrimal bones ossify each from a single (•(•lit*!-, uhirli a|»|M':ir- in the ci^dith week.

The malar i- o--ifn d in nicniKrane from three nuclei, the ppKM-- iMMiniiiiiir in tlic eighth week.

riic palate bone is iorinr<l in ninrous membrane fn>m a -iiw^lc cciitf r whirli i- .-itnal<_'(l at the jniiction of the vertical and tlic horizontal pl.itc^.

riic vomer d<'V('l(»|>s from two center- of ossification which a|>|H':ir at the hack |)art of the cartilaLrinotis nasid septum. Ivi'h <'eiiirr Liive- rise to a laniina ol' hone, the two laminae

rMdually iinitiiiLr with each other from behind forwanl, and

eiiil>r:uinL'" Ix'tween them anteriorly the septal cartilage.

The vomer and the palate bone are examples of the formation of JM)iie in iniieoiH in<'inl>rane. The centers of ossification lir-t aj)peMr in the eighth week in each ca-e.

'i'he skeleton of the visceral arches includes the upi>er and lower niaxilhe, the liyoi<l Ixnu* with a ])art of the styloid |)rocess, tlu? ear ossicles, and the j)alate bones. The ]>alate hone- have heeii referred to above. These hones of the visceral-arch skeleton are partly primordial and partly membranous.

The snperior maxilla comprises two parts, the superior maxilla proper and the intermaxillary bone. While these intimately unite in man, in some animals, as the dog, they are permanently distinct, the intermaxillary lK)ne constituting the important and conspicuous premaxilla of the dog. The superior maxilla ossifies in membrane — within the membranous maxillary process of the first visceral arch — from an uncertain number of centers. It seems probable that there are five nuclei of origin, one for the palate process, one for the malar or external part of the bone, one for the portion internal to the infra-orbital foramen and a part of the nasal wall (orbitonasal center), one for the part of the bone between the frontal process and the canine tooth, and one for the premaxilla. The formation of the antmm begins in the fourth month by the development of a recess or fossa on the inner or nasal wall of the bone.

The palate process is formed by the growth, on the inner aspect of the bone, of a shelf-like projection which advances toward the median line until it meets and unites with its fellow of the opposite side (Fig. 172j. The horizontal plate of the palate bone develops similarly and very shortly after, and thus is produced the hard palate, which separates the nasal chambers from the mouth. The two halves of the hard palate unite first in fnmt, their union being completed by the twelfth week. If union is incomplete, the anomaly of deft-palate results. The intermaxillary segment begins its development in the seventh or eighth week upon that part of the nasofrontal process which lies between the nasal apertures. In the fifth month the intermaxillaries fuse with the maxillse, the line of union being indicated by a suture which is apparent upon the oral surface of the palate processes. The intermaxillaries contain the germs of the four incisor teeth. As previously mentioned, deficiency of union between the maxilla and the intermaxillarv results in the deformity of hare-lip. Obviously, the hiatus in hare-lip will be found to be not me<lian, but lateral, corresponding to the position of the line of normal union.

Tliti lower jaw or mandible is intimatcl}' associated ia its (K^vclopriiuiit witli that of the malleus and incus of the middle (•(ir. Inasmuch as thoric thrt-e Inmes are dilfcrentiatcd from ihi! it:irti[:i;riiioiis anil inumliniiioiis visceral skeleton of the first vi.-wrrtil arch it is di'sir.ibk< to consider their develop IIHlll toj^tiuT.

As dfscrilied aliove, the inenihmiiotis jaw-arches form the hiliral and Idwit Iiniiiidtirics of the month-cavity, the first vis<i'ral arch dividing into the nuixillary process and the iiuiiidihiihir :m-li. Thenr apjH'iirs in the mandibular arch a bar of cartilage which abuts liy its jmiximal extremity upon th Iter wall of the au<litory labyrinth. This cartilaginous

" r'ii;)il<'t'ii n-i-Lka nld with the 1til. 1'lii- liiniT Jaw lomewhU 'li i.'iiti'iiil:> III till' niHlkiii. The lyuitaiiicu!! iKvikible: Aa.nwlfiirlilnip-. .Vt: uk, biiiijr lower

>. iia*

,,:,h,l,..iu.dm.u,

orliim, Meckel's cartilage (Fifr.

•.iNii.ial pi.ve. whifh is called,

- |>al:ito<|ii:iiIi-:itLnti. From the

Hie {i:<[iili>|ilery[ruiil process.

grows toward the roof of the mouth-cavity and becomes a separate segment. The piece of cartilage remaining, which represents the proximal end of the original bar, undergoes ossification, becoming the incus (Fig. 185, am). The posterior or proximal extremity of Meckel's cartilage, becoming a partly sej)arated cartilage, the articulare, ossifies to produce the mallens (Fig. 185, ha). Though the form of the malleus is recognizable, it is still in direct continuity with Meckel's cartilage. In the opposite direction it is articulated with the incus. As the tympanic ring develops, and the interval below, between tliis ring and the petrosa, is gradually narrowed to the petrotympanic or Glaserian fissure, the malleus comes to He within the tympanic cavity, being continuous, through the fissure, with Meckel's cartilage. Upon the separation of the malleus from the cartilage of Meckel, the long process of the malleus represents the former bond of union and therefore occupies, in the mature state, the Glaserian fiasure. The joint between the malleus and the incus represents the primitive vertebrate jaw articulation. In the shark, for example, the mandibular joint is between the two pieces into which the cartilaginous bar of the first visceral arch divides — that is, between the palatoquadratum and the representative of Meckel's cartilage, the mandibulare. In mammals, however, the malleus, as we have seen, loses its connection with the mandible, the joint between the latter and the skull, the temporomaxillary articulation, being secondarily acquired in a manner to be pointed out hereafter. While the malleus develops for the most part by ossification in cartilage, its long process develops in membrane as a small covering or dermal bone, the angulare.

The membranous lower jaw with its enclosed bar of cartilage becomes osseous, not by the ossification of the cartilage, but by the development of a casing of bone within the surrounding membrane. In other words, the lower jaw develops chiefly by the intramembranous method of boneformation. Several centers of ossification appear, and from these the process of bone production extends rapidly, forming, by the fourth month, a covering or dermal bone, the dentale (Fig. 185, uh)^ which is situated mainly on the outer side of Meckel's cartilage. A smaller plate appears on the inner side. Thus the cartilage comes to be surrounded by an irregular cylinder of bone. The cartilage of Meckel plays a comparatively unimportant part in the ossification of the lower jaw-bone and begins to degenerate in the sixth fetal month. Its distal extremity, however, undergoes ossification, thus aiding in the formation of a small part of the mandible near the symphysis; while a posterior segment, with the fibrous tissue encasing it, which extends from the temjx>ral bone to the inferior dental foramen, persists as the internal lateral ligament of the lower jaw. With these exceptions, Meckel's cartilage entirely disappears. The angle of the mandible and a small part of the ramus are also ossified in cartilage, which latter is developed independently of Meckel's cartilage. From the posterior part of the dentale the condyloid process develops and becomes articulated with the glenoid fossa of the temporal bone, thus establishing the temporomaxillary articulation. This joint, as previously stated, is a secondary one and replaces in mammals the primitive articulation between the mandibulare and the palatoquadratum of the lower vertebrates.

At birth, the two lateral halves of the inferior maxilla are united at the symphysis by fibrous tissue ; bony union occurs during the first or second year after birth.

To summarize, the inferior maxilla develops as a part of the visceral -arch skeleton and is chiefly a covering bone, since, with the exception of the angle, a portion of the ramus, and a small part near the symphysis, which are of cartilaginous origin, it is formed by the membnmous method of ossification. The two other products of the mandibular arch, the malleus and the incus, are ossified from cartilage, with the excej)tion of the processus gracilis of the malleus, which is of membranous origin.

The development of the hyoid bone, of the styloid process of the temporal bone, and of the stapes was referred to in considering the c4irtilaginous visceral-arch skeleton, but for the sake of clearness and completeness it may not be amiss to repeat, in this connection, Bome points previously mentione<!.

The membranons ulterior hyoid or second Tisceral arch, at a certain stage of development, presents, in ita interior, the dorsoventral cartilaginoiiii bar known as Keichert's cartilage. This is parallel with Meckel's cartilage, and, like it, is in contact by its dorsal or cranial end with the outer wall of the auditory labyrinth. A shorter bar of cari:ilage appears in the third visceral arch, which latter is known also as the posterior hsroid arch. Together, these two cartilaginous elements furnish the stapes of the middle ear and the hyoidean apparatBS, the latter consisting of tlic hyoid bone, the stylohyoid ligaments, and the styloid processes. In man the

llfOldeaii apparatuB and Inryni at dog.

hyoidean apparatus is somewhat nidlmontary, but in the dog and many other mammals it is present in its typical form (FIr. 18G). In such animals the stylohyoid ligament of human anatomy is roprescntrd by a b()ne, the epihyal, the hyoid bone being, therefore, connected with the skull by a series of small bones artieulatod with earh other. AH the elements of the hyoidean apparatus, ^ave the IwmIv and the greater cornna of the livotd liono, are produced bv Reichort's cartilage ; the hyoid Iwdy, known in comiKirutive anatomy as the basiliyal, .tihI the greater eornua, or the thyrohyals, ossify from the cartilage of the third arch, the cartilage for the body being a median unpaired segment known as the copula. Beicliert's cartilage undergoes division into five s^ments. The segment at the cranial end, upon ossification, becomes the stapes/ This ossicle, by the closing of the walls of the tympanic cavity, is isolated from the other segments. The second piece, the tsrmpanohyal, ossifies to form the base of the styloid process and ankyloses firmly with the temporal bone at the point of junction of the periotic portion of that bone with its tympanic plate. The third portion, the stylohyal, forms the lower part of the styloid process. It undergoes ossification later than the tympanohyal and does not acquire osseous union with it until the time of adult age. It sometimes remains separate throughout life. The fourth segment, the epihyal, does not even become cartilaginous in man, but remains fibrous, constituting the stylohyoid ligament. In most mammals it ossifies, to form a distinct bone, the epihyal. The ventral extremity of the cartilage of Roichert, the ceratohyal, produces the lesser cornu of the hvoid bone.

THE DEVELOPMENT OF THE APPENDICULAR SKELETON.

The upper and lower limbs articulate with the trunk through the modium respootivcly of the pectoral and pelvic jj^irdlcs, tho fornicr being constituted bv the scapula and the elaviele, and the latter by the ossa iiinoniiiiata. As in the ease of the axial skeleton, the hones of the limbs in their develo]nu(jnt ])ass sueeessively through a nienibnmous and a cartilaginous stage.

The general (h^velopnient of the upper and lower extremities is deseribed in a later section. As stated in that account, each linii)-bud is to be regarded as an outgrowth from, or as eorresj>ou(ling in posili(ni to, several primitive segments, the tissue composing the little bu(l-lik(» ])rocess subsequently differentiating into the muscular, cartilaginous, and connectivetissue elements (►f the member. The origin of each limb from more than one primitive segment has been established

^ Si'u ft)ot-n()te, i>:igt.' J 15).

chiefly by eml)ryological investigations upon the lower vertebrates, and is borne out by the fact that each extremity receives- its nerve-supply from a series of spinal nerves instead of from the nerve-trunk of any one segment.

The Development of the Pectoral and the Pelvic Girdles. — The pectoral or shoulder girdle consists in its earliest stage of a pair of curved bars of cartilage, each of which is made up of a dorsal limb occupying approximately the position of the future spine of the scapula and approaching but not touching the spinal column, and a ventral segment lying near the ventral surface of the trunk. At the angle of union of the dorsal and ventral parts is a shallow depression, an articular surface, which represents the point of articulation with the future humerus.

The scapula is developed, except its coracoid process, from the dorsal part of the primitive shoulder-girdle. This soon acquires a form resembling that of the adult scapula with the infraspinous portion of the bone very much shortened. Ossification begins at the neck of the scapula about the eighth week, and in the third month extends into the spine. The ventral part of the cartilaginous shoulder-girdle extends almost to the median line of the chest-wall. It divides into two diverging bars, the lower one of which undergcx^s ossification in birds and in some other vertebrates to form the conspicuous coracoid bone. In mammals, however, it aborts and gives rise to a smaller element, the coracoid process of the scapula. At birth the human scapula is but partially ossified, the coracoid process, the acromion, the edges of the spine, the base, the inferior angle and margins of the glenoid cavity being cartilaginous. The coracoid process ossifies from a single center and acquires osseous union with the body c>f the bone at about the age of puberty. The acromion ossifi(s from two or three nuclei and joins the sj)ine between the twenty-second and twentyfifth years. Still other centers of ossification ai)pear from time; to time. Thus there is an accessory center for the base of the cf)racoid and the* adjacent part of the glenoid cavity, and one at the inferior angle of the hone, from which latter ossification extends along the vertebral border.

The clavicle does not develop from the primitive shouldergirdle, but is formed in membrane, for the most part, as a dermal bone. 1\^ ossification begins in the sixth or seventh week, before that of any other bone in the body. Subsequently, cartilaginous epiphyses are added, one at each end. It is by means of the epiphyses that the bone grows in length.

The cartilaginous pelvic girdle consists of a pair of cartilages, which are united with each other by their ventral extremities, and each of which, by its dorsal end, is articulated with the sacral region of the cartilaginous spinal column. At about the middle of each cartilage, on its outer surface, is a depression representing the future acetabular fossa. Anterior to the depression is a large ajjerture, the thyroid foramen, the upper and lower boundaries of which are respectively the pubic and ischiatic rwls or bars, which make up the ventral portion of the cartilage, while posterior to the fossa is the iliac segment, which has a somewhat irregular jilatc-likc form. Ossification Ix'^ins in the third month, ]>roccc(Iiii<2; from three centers, one for each of the tlire(? divisions of the innominate l)one. At the time of birth a large pro|)oriion of the orijxinal cartilage is still present, the os pnl>is, the ischium, and the ilium being separated from each other np to the age of puberty by strips of cartilag(». The ischium and the pubes unite first, and later acquire osseous union with the ilium. In addition to the three ])rimarv centers of ossification, other and secondary nuclei apj)ear at a later date in the crest of the ilium, the tuberosity of the ischium, and in i\w various spines and tubercles.

The skeleton of the free portions of each extremity, consisting at first of a continuous mass or rod of partially metamorphosed mesenchymal tissue, undergo(»s division into segments which represent the skeleton of the arm or of the thigh, of the forearm or of the leg, and of the hand or of the foot. This segmentation corresponds with that of the entire mass of the limb, both as to extent and order of appearance (see page 406). Nuclei of chondrification now appear, one in the center of each skeleton-piece, from which cartilage formation extends toward either end. The several cartilaginous elements thus pnKluced present approximately the respective forms of the future bones. The larger cartilages are present in the upj)er extremity in a six weeks' embryo, but not until somewhat later in the lower limb. All the bones of the extremities are of endochondral origin.

The long bones develop in a fairly uniform manner. The shaft or diaphysis ossifies from a single center, while the two epiphyses each present several centers. The centers for the diaphyses appear at about the eighth week, ossification proceeding at such rate that at birth only the ends of the long bones are cartilaginous. The centers for the epiphyses appear at various times after birth. Osseous union between the diaphysis and the epiphyses does not occur until the growth in length of the bone is completed. As the details conceniing the time of appearance and the number of these centers are to be found in the text-books of anatomy, they are omitted here.

Each bone of the carpus and of the tarsus ossifies from a single center, except the os calcis, which has two ossific nuclei. The bones of the carpus are entirely cartilaginous at birth, their ossification beginning in the first year with the appearance of a center in the scaphoid. The pisiform bone is the last of the series to ossify, its ossification beginning in the twelfth year.

The bones of the tarsus begin to ossify earlier than those of the carpus. The os calcis and the astragalus present osseous nuclei in the sixth or seventh ft^tal month, and the cuboid shortly before birth. With thos(^ excej^tions the tarsal bones undergo ossification between the first and the fourth years.

The metacarpal and the metatarsal bones and the phalanges present each a center of ossific^ation for the shaft and one epiphyseal center. In the case of the phalanges and of the metacarj)al bone of the thumb and of the great toe, the epiphyseal center is at the proximal extremity, while in the

n'inaininj: nu'tatarsiil anhaft begins in the eighth or ninth wwk of fetal life; <»f the epiphyses, n(»t until scvenil y«irs after hirtli. The development of the ungual or distal phalanges — of the hand, at least — is jH-euliar in that the ossification l)egins at the distal extnMuity, instead uf in the middle uf the shaft.

THE DEVELOPMENT OF THE LIMBS.

The linihs of vertehnites develop fi-om little bud-like i)r(KH'sses (Fig. iVl) liiat spring from two lateral longitudinal ridges, situattnl one on eaeli side of the body. Thes(» ridges are not exactly i^anillel with the nuMhau ]>hine of the body, but converge somewhat toward that jilane as they api)roaeh the «uidal end of the embryo. It results from this circumhiancc that the jMistcrior limbs are jilaeed chaser together than the anterior. In man, the limb-buds ai)pear soon after the thin! wivk. Kju'Ii bud contains a basis (»f j)rimitive eonnivtivc tissue oontributt»<l by several somites, as >vell as muscubr <tnietnrt\ whioh is th(? ofi«hoot from the muscle-plates 0-' J 1.^ numlKT of primitive si^gments.

• Thr assumption of the origiu (►f each limb-bud from more

i «r^mitivc socmont is borne out by the nerve-sup])lv Th' fu]lv-l>"^*^' l"»l^' ^*"** extremity being innervated by ' ' ' ^ ■ •' <nir.-il nerves (compare page ;}GS). The eon.>a<m of the limM)ud i)ro(iuces the bony structures while the i»utgn)Wths from the mustrle-plates \r.y mnsriilntun*. Previous reference has been -.. t.^ ihe work of Banleeu and Lewis on v^ ihr limbs. Aci'ordiug to their findings,

T^y fVTcnd into the limb-buds, i)ut the limb .^ f«rtni the mestMichymal core of the bud.

""""'* -,,*; thf bnJ for the arm is at first ojiposite

i.\'/*!ol' the third ('crvical >cgm«'nt, ^. N.-^v* 10 its adult position ; and that the hud for the leg, at first attached at the region of the lower four lumhar and first sacral myotomes, extends to include the first lumbar and the second and third sacral segments, assuming later a more caudal position.

In the fifth week each limb-hud becomes divided, by a transverse groove, into two segments (Fig. 59, 12, 13), of which the distal part becomes the hand or foot, while the proximal j>ortion very soon afterward divides into the forearm and arm or leg and thigh. Even as early as the thirty-second day, the digitation of the limb-buds — in the case of the up]>er extremities — is indicated by four longitudinal parallel lines or grooves on the distal extremity of each (Fig. 59, 14). By the conversion of these grooves into clefls, the fingers appear, in the sixth week, as separate outgrowths. The development of the upper extremities precedes that of the lower by twelve or fourteen days, so that, when the fingers are present as distinct projections, the toes are just being marked off in the manner noted above for the fingers. The toes begin to separate, by the deepening of the intervening clefts, from the fiftieth to the fifty-third day. By the end of the eighth week, the fingers are perfectly formed, with the exception of the nails. The nails have their beginning in the seventh or eighth week, in little claw-like masses of epidermal cells, which are attached to the tips of the digits instead of to the dorsal surfaces. Subsequent transformations result in bringing the nail into its normal position on the dorsal surface of the distal phalanx. The nails are well formed by the fifth month, at which time the covering of modified e])idermal cells begins to disapi>ear. The extremity of the nail, however, does not break through so as to project beyond the finger-tip until the seventh month. A more complete account of the development of the nails will be found in connection with the origin of the skin (page 270).

The Position of the Wmbs.— The paddle-like limbbuds at first project laterally almost at right angles with the axis of the trunk. At this time the future dorsal surface of eacrh limb looks toward the back of the fetal Ixxly (dorsad), the future flexor surface toward its anterior aspect (ventrad), while the first digits — the future thumb and great toe — and consequently the radius and tibia, occupy the side of the member that is directed headvvard or cephalad, the future little finger and fifth toe with the ulna and fibula looking caudad. As the limbs enlarge and differentiate into their respective segments, they apply themselves to the ventral surface of the body, this change in position being facilitated by the occurrence of the future elbow- and knee-flexions, which cause the flexor surfaces of the forearm and leg, respectively, to approach the corresponding surfaces of the upper arm and thigh. At abf)ut the same time, the distal segments, the hand and foot, become bent in the opposite direction, producing the condition of the limbs that is permanent in the Amphibia — that is, the condition in which the dorsal surface of the proximal segment of the limb faces in the same direction as the dorsal surface of the trunk, while the middle segment is flexed and the distal is extended. To establish the permanent condition of the human limbs, there occur an outward rotation of the arms and an inward rotation of the lower extremities, on their long axes. The thumb and nulius, therefore, instead of looking cephalad, are now directeil dorsad — with the forearm in the supine jxjsition and the arm outstretched — or laterad, away from the median plane of the body, if the arm hangs by the side in the anatomical j)osition. By the inward rotation of the lower limb, the great toe and the tibia come to lie toward the median plane of the body, cjiusing the extensor surface to look ventrad, the flexor surface, dorsad.

TABULATED CHRONOUXIY OF DEVELOPMENT.

PertillMtioS!"*

STAGE OF THE OVH«. FiiCT Weik. Second Week.

Obanctwi.

SegmenUtlon of fartlllied while pauine along ovl Great Increase In tl«. Cella of loner cell-maw rearranged lo form cnto cell* of Rauber.

eytlum (.--3 dar).

Ovnm Id ntema, embedded

C°ortSn'and il> Tilll (FlgJ 49). YapeularitalloQ of Fliorlau and ila villi.

Yalk-uc partly formed.

ViLtenUr

rf"yolk-wc. ""

"Vy'SS,.

Oral pit (12thMl«h day). «m-lmrt partly «.i-.n.ted from yHlk-BBc.

•*?SS!i.«"'

STAGE OF THE EMBKYO. TuiRD WEEK, Fotiavn W««.

Ouiarml Cbuutan.

Body of embryo indicated.

VUcural arche. and clefia

NMo^fciltkr"™™ AllBiitoiCBtnlklFlgisTt. lHjllnctton betweuii chorion Jevc and rhorion fWn

Marked flexion of body (21irt to lOd day): sradual uncoiling after aSd day.

Vlacerararchea and jolk-aac

to flatten. CephBllo Bcxurea.

"nsas..

Heart wllb einitle caTlty present, aooo dlvldltig Into

VlaCBral-areh -veueli beglu to appear.

IMviilon of atrium begini.

DlceiUTa TlyaMm.

(5ut-lracl a strBlght lube connetted wlih yolk-«ae by a

Anal Plate.

Alimentary canal ptcocnla pharvni. euipbapu, atomach. and intwliue.

Fancrcaa bvguii.

l.lver-dlTPrileulum divides,

Blle-diicta acquire himlna.

brealiB down,

-C£'

tral wflU of esophagus, arterward becoming a

Pulmonary anlage blftircall's, the two poaches being eonne-ted by « pedicle,^he prlraltlTC irwhea. with the pharynx.

"XS^""

Wolffian bodies reeoBnlsuble.

BUn.

SeKmenlation of paraxial mesoikTm,

HeurBl canal : Its cells sho*

El obi oats and Kerm-cellH Fourth ventricle Indlcaled. Fore-bralQ mtd-brBln, and hind-brain -reniples. s-kiu illvidlns Into five vealelcs.

Auditory pll followed by ollc

nlfaclory plalM. Optic vL-hlclcflbi-frtn,

I'utL-plite.

Vettmu 87item.

thicken. Yenlral roots of aplual

Anterior' lobe of hypopbrila

begins.

Smel&l Bsnae

OTgMM.

Ollc vealcle with recenu

NHMlplu dl'itlnd. lJl4lc veaicic atalked and tmuBformetl IntooptlccDp,

MOBCular ByaCem.

Bkflleton Mid

Limbi.

iiicsodcrm. ft-Km.nlallon of paraxial

Somites or primitive acf

Hyiilomea.

S.iniltcs or prlmlllre aeg Sk"ci(.'t.ien.ina ahcath of

phorrla. Llmb-hiidii Biijarent (aboot

2lst day).

TEXT-BOOK OF EMBRYOLOGY.

411

Tabuulted Chronoloot of Development (Oontinued),

STAGE OF THE FETUS.

Fifth Week.

Sixth Week.

Body shows dorsal coDcavity in neck- NasofVontal, lateral nasal, and maxil

region

Globular and lateral nasal processes. Lacrimal groove. Third and fourth gill-clefts disappear in

sinus prscervicalis. Umbilical cord longer and more spiral. Umbilical vesicle begins to shrink. Length of fetus 1 cm. ({ inch). Larynx indicated.

lary procesnes unite. I'mbilical vesicle shrunken. Amnion Larger.

Primitive aorta divides into aorta and I)ulmonary artery.

The only corpuscular elements of the blood during the first month are the primitive nucleated red blood-cells.

Vitelline circulation atrophic and replaced by allantoic circulation.

Intestine shows flexures, notably the U-loop, inaugurating the distinction between large and small bowel.

Anal pit.

Right and left bronchi divide into three and two tubes respectively (5th to 7th week).

First indication of teeth in the form of the dental shelf.

Submaxillary gland indicated by epithelial outgrowth.

Duodenum well formed; caecum; rectum (end of week).

Larynx indicated as dilatation of proximal end of trachea.

Arytenoid oartiliiges indicated (though not cartilagiiiouM).

Thyroid and thymus bodies begun.

Genital ridges appear on wall of b<»dycavity and soon t>ecome the indifferent genital srlands.

Ducts of Mtiller apfiear.

(ienital tubercle, genital folds, and gen- ' ital ridge (external genitals). '

Epidermis present as two layers of cells.

CoIIh of oiiti>)-t>late proliferate and gradually hpreau out beneath epidermis.

Olfactory lobe begins.

Arcuate and choroidal Assures on mesial surfaces of fore-brain vesicles.

Cells of central canal of cord ciliated.

i Ridge-like thickening of roof of mid! brain.

Membranes of brain and cord indicated. I*inenl body U;gins. Dorsal roots of spinal ner\'es. Home tracts of spinal cr>nl indicated, and its lumen alters (Fig. 139).

Semicircular canals indicatcnl. Semicircular canals.

Eyes begin to move forward from side Concha of external ear. of head. ' Outer flbrous and middle vascular tu 1 nicR of eye. Eyelids

I Mandibles unite (a')th day).

, Meckel's cartilage.

I Limb-buds segment.

I Digitation indicated (32d day, for hand.

I^wer jaw begins to ossify.

Clavicle iHJgins to ossify.

Rilw l>egin to chondrify.

Bodies of vertebra: are cartilaginous.

Fingers as sefMirate outgrowths.

412

TEXT-BOOK OF EMBRYOLOGY.

Tabulated Chronology of Development {Contiuued).

•

STAGE OF THE FETUS. Seventh Week. Eighth Week.

General Characten.

Fetal body and limbs well defined (Fig. 64).

Head less flexed.

No longer any trace of syncytium on dccidua vera.

Head more elevated (Fig. 65). Free tail begins to disappear. Subcutaneous lymph-vessels

present. Oils lining the coelom are

true endothelium.

VaBcular Bystem.

Interventricular septum of heart completed, tne heart now having four chambers.

Other corpuscular elements added to blood during second month.

Dlgeetlve System.

Transverse colon and descending colon indicated.

Parotid gland begins.

True endothelium lines the body-cavity.

Gall-bladder present (2d month).

Anlage of spleen recognisable (2d month).

Respiratory System.

Median and lateral lobes of thyroid unite.

lArynx begins to ehondrify. Formation of follicles of thymus.

Oenlto-nrlnary System.

Maximum development of Wolffian body.

Mailerian ducts unite with each other. Genital groove.

Bladder present as spindleshaped dilatation or allantois.

Suprarenal bodies recognizable.

SUn.

Nails indicated by claw-lIkc masses of epithelium on dorsal surfaces of digits.

Corium indicated as a layer of spindle-cells beneath epidermis. Development of mammary glands began.

Nervous System.

Fore-brain vesicles increase in size disproportionately. Cerebellum indicated.

Sympathetic nerves discernible.

Special Sense Organs.

External nose definitely formed (Fig. 171).

Lens-capsule.

Palpebral conjunctiya separates from cornea.

Muscular System.

Muscles begin to be recognizable, though not having as yet the characters of muscular tissue.

Ossific centers for vertebral arches and for vertebral l»odies; ossiflc renters for frontal bone and for sciuamosa.

Membranous primordial cranium begins to ehondrify.

Claw-like anlages of nails.

Skeleton and Limbs.

Ribs begin to ehondrify. Centers of ossification of bestsphenoid, of greater wings. of nasal and lacrimal bones, of malar, vomer, palate, neck of scapula, diaphysos of long bones and of metacar)>al bones. Fingers perfectly formed. Toes begin to seiMirate (53d day).

Tabulated Chronology of Development (Continued),

STAGE OP THE FETl'8. Ninth Week. Third Month.

Weight, 15 to 20 frrams ; length, 25 to 30 Weight (end of month), 4 ounces: length,

mm. (1 to Ij inchefi). 2) Inches.

Hard palate completed. - At first chorion Icve and chorion fron • Free tail has disapfteared. dosiim prt'oent : later, formation of

Differentiation or lymph-nodes begins placenta (see second frontispiece). (O. Schultze). (Uoaea divided. I

Pericardium indicated.

Placental system of vessels. Blood-vessels fienetrate spleen.

Anal canal formed by division of cloaca. Mouth-cavity divided fh)m nose (end of

(Anus opens at end of '2d month, ac-, cordiuK to Tourncux.)

month). Soft (Mlate completed dlth week). Papillie of tongue. Evagination for tonsil. Intestine begins to rect»de within abdomen (10th week). Rotation of stomach. Vermiform api>endix as a slender tube. Omental bursa, (tastric glands and glands and villi of intestine fairly well formed (lOth week I. Liver verj* large. Peritoneum has its adult histological characters.

Epiglottis.

External genitals begin to show distinctions of sex.

Ovary and testis distinguishable fVom each other.

Kidney has its characteristic features.

Urogenital sinus ac<iuires its own aperture by division of cloaca.

Union of ti-stis with canals of Wolftian

lM)dy conn>lete. Testes in false jx-lvis. Ovaries descenM. Prostate K*gun d'ith week).

r«>riuin projK»r present as distinct layer. | Nails not (juite iH»rfeetly fi>rmed. Hepinning Jif <ievelopiiient of hair as ' solid ingrowths of epithelium.

Corpus striatum in«licated. j Corpora quadrigemina represented by two elevations on mid-brain roof.

Cerebrum covers inter-brain. Fornix ' and corpus callosum iK'gun. Fissure ' of Sylvius. Calcarine fissure. Crura cerebri. Kestiftirm bodies. Pon.s.

! External ear indicated (Fig. Ciliary processes intlicated.

170).

Eves nearly in normal iK>8ition. Eyelids begin to adhere to each other.

\

Centers of ossillcation of presphenoid. Ik'jrinningossiticationof occipital bone. " of les.«ier wings of sphenoid, and t)f of tympanic, of spine of scapula, of shafts of metatarsal bones. ossu innominata. '

Cnrtilacinovis arches of vertebrse close. Limbs have definite shape ; nails almost |)erfeetly formed.

Tabulated Chronology op Development (Coniinwd).

General CliaracterB.

I

Vasciilax System.

Digestive System.

Respiratory System.

Oenito-urlnary System.

SUn.

Nervous System.

Special SenBo Organs.

STAGE OF THE FETUS. Fourth Month. Fifth Month.

Weight, 7j ounces: length, 5

inches. Head constitutes about oneI quarter of entire body.

Enamel and dentine of milkteeth. Germs of permanent teeth tl7th wk) : (for 1st molar, 16th wk). Muscularis (longitudinal and cirt>ular) of stomach and esophagus. Intestine entirely within abilomen. Acid cells of peptic glands. Malpighian I bodies of spleen. Anal ' membrane disappears.

! Cells of tracheal and bron' chial mucous membrane ciliated.

Weight. 1 lb. : length, 8 in. Active fetal movements begin. Two layers of decidua (Mialesce, obliterating the space between vera and reflexa. Lymphatic glands begin to appear.

Heart very large.

Salivary glands acquire lamina.

Villi of large intestine begin

to disai>piear. i Liver very large.

Meconium shows traces of bile (sometimes early in fourth month).

Sexual distinctions of external organs well marked. Closure of genital farrow. Scrotum. Prepuce. I*rostate well formed.

Pftpilln? of corium. Subcutaneous fat first appears. I^iiiugo or embryonal down on scalp and some other parts.

rnrieto-orcipital fissure.

Distinction between uterus

and vagina. Hymen begins.

j CorfHtra alhicantia. Tmnsvei

»rse fibers of p<m«.

Middle i>eduncle8 and chief fissures of cerebellum.

Spinal cord ends at end of riM'cyx.

Deposit of myelin on fihrrs of |Mist«Tior* roots, extending to liurdaeh and (ioll.

I

Panniculus adiposus. lanugo more abundant. Sel>aceous and sweat-glands iH'gin.

Fissure of Rolando. Body of fornix and corp. caliosum. I>on>:itudinal fibers in crura cerebri. Superior peduncles. A nterior pyramids of medulla. Chief transverse fissures of lateral lobes of cerelwllum. Deposit of myelin completed for tract of (roll and later of Burdach. and for short commissural fibers (Tourneux).

Kyolids and iiostriN closed. ("urtiljiKcof Kustnchiun tnlK?.

Orj^m of Corti indicated.

Muscular

Difiirentiation of muscular

System.

tissue of arms.

Skeleton and

Os»i<'(ms crntcr for inteninl

(>s*!ifl<«tion of stapes and petriisji. Opisthotic and prootic apjH-ar. Ossificati<m

Limbs.

ptrrvjroid |»ljite.

Antnnn of lliirhmore b^'cins.

<Ksin«'Mtioii of malleus and

iM'irins in middle and infe

incus.

rior turt)inals and lateral

Tnav»jr«i of t>thmoid. Internal pt<'ryiroi<l plate Aisea witlj •'Xternal. Intermaxillarifs fuse with maxilla. I^'us longer than arms.

Tabulated Chronology of Development (OonHwued),

STAGE OF THE FETUS. Sixth Month. Seventh Month.

Weight, 2 poands ; length, 12 inches. Vemix ca«eo«a begins to appear. Amnion reaches maximum size ; amniotic fluid of maximum quantity.

Weight, 3 pounds ; length, 14 inches. Sur»ce less wrinkled owing to increase of fiit.

Peyer*s patches. I Meconium in large intestine.

Trypsin in pancreatic secretion (fifth ■ Ascending colon partly formed, or sixth month). Csecum below right kidney.

Air-vesicles of lungs begin to appear.

Walls of uterus thicken.

Vernix caseosa begins to appear. Eyebrows and eyelashes begin.

Testes at internal rings or in inguinal canals.

Epithelial buds for sebaceous glands acquire lumina. Branching of cords of milk-glands. Eponychium of nails lost; nails said to break through, lanugo over entire body.

Collateral and calloso-marginal Assures.

Body of fornix and corpus callosum complete.

Hemispheres of cerebrum cover midbrain.

Cerebral convolutions more apparent.

Cori>ora nuadrigeraina.

Myelination of fibers of direct cerebellar

tracts. (Crossed pyramidal tracts not

until after birth.)

Lobule of ear more characteristic.

Lens-capsule begins to acquire trans- , mrcnry. Kyelids permanently open. , rupilliiry membrane atrophies.

DiflTorontintion of muscular tissue of , lower extremities.

i I^esser winps unite with presphcnoid. i Basisphenoid and presphenoid unite ' Mockel'H cartilHffo h<»cin« to rctrogrn<lo. I (7th or blh month). ' Ossific nuclei of os calcis and astragalus.

Tabulated Ciironoloot of Development (Conduded),

STAGE OF THE FETUS. Eighth Month. Ninth Month.

General Charftcters.

Weight, 4 to 5 pounds ; length,

16 Inches. Body more plump.

Weight, 6 to 7 pounds ; length,

20inches. Umbilicus almost exactly in

middle of body.

Vascular System.

9

Digestive System.

Ascending colon longer. Caicum below crest of Ilium.

Meconium dark greenish.

Respiratory System.

Oenito-orinary System.

Testes In inguinal canals.

Testes in scrotum. T^bia m^Jora in contact.

SUn.

Vemix caseosa covers entire

body. Skin briehter color. Lunug(j begins to disappear. Nails project bi'yond hnj^er tii>a. Increase of subcutaneous

fat.

Lanugo almost entirely absent.

Galaetopherous ducts of milk-glands acquire lumina.

Nervous System.

Spinal cord ends at last lumbar vertebra.

Special Sense Organs.

Ossification of bony lamina spiralis and of modiolus.

Neuro-epithelial layer of retina completed; macula still absent.

Choroidal fissure closes.

Muscular System.

Skeleton and Limbs.

Ossification in lower epiphysis of femur, sometimes also in ui>i>er ei>iphy8e8 of tibia an(i humerus.

Tyinpanohyal begins to ossify.

Ossitlc nuclei for body and great horn of hyoid bone. 1

---

INDEX.

Abdominal cavitv, development of,

215 Accessory sii])nirenul oi*};ans, 242

thyroid. 227 Acetabiilur fossa, 404 Achoria, 1)4 Achroiuatin, 27 Acid colls, formation of, 200 Acoustic gaii)j:lioii, ;J21 Acusticofacial jraiijilioii, 321 Adaniantoblasts. I'.VJ Adenoid tissue. develo|>ineut of, 129 Adipose tissue, formation of, 12G After-birth, 104 After-brain, 2S7 Age of fetus, estimation of, 122 Air-chambtT of hen's e;;i;, 29 Air-sjK's, development of. 2"2o Ala* of n<)se. <h'volopment of, 3<)2 Aiiir lamina. 290 Alecithal ova, 2()

Alimentary c^inal, develoi)ment of, Ls5 * diflcrentiatiou into separate region^. 11>7 histolojiical alteratitms in. 20.")

tract, alteration in position of parts, 202 in«.'rease in length of. 201 Alisphenoids. .'{91 Allantoic arteries. 90, 104

circulation. 90 formation of, 103

stalk. .N'>

veins, im, 104 Allantois, ^9, 190. 2.")

function of. JM)

resi)iratory function of. 200 Alveoli, ]>ulmonarv, development of,

225 Ameloblasts. 1.39 Amnion, HI, t<'2

false, si

of man, >^^ Amnion-fold, 80. J^l. R3 A mil iota. H3 Amniotic cavity, 54, 84. 5i5

lluid. 85. SO functi<ui of, J^O

suture, S3 Amphibians, blastula of. 51 Amt)hioxus. blastula of. 50

.skehjtal apparatus of, 372

• >7

Ampull(e of semicircular canals, development of, 34rt

seminal, 240 Anal canal. 257

membrane, 195

l»late, 195 Anamnia, Si Angioblast. 147 Animal ]Mjle. 27 Animalculi^ts. 18 Anhige, 175

median, of thyroid body. 228 Annular sinus, 179 Annulus ovalis, 157 Anomalous arrangements of aortic

aivh, lOs Anterior chamber of eye, 342

nares. development of. 1 10, 3«U»

pyramidal tmcts of medulla, development of, 2JK> Antitragus, formation <»f, 35*< Antrum of liighmore. di'velopment

of, 301 Anus, development (»f. 195

imi)erforate, 1!»7 Aorta, caudal, KJO

develo]iment of. 159

l)rimitive, 151. 1(J5 Aortic arch, anomalous arrangements t.f, HW

arciies. 105

septum. 159 Api>enda«j:es of skin. 27n Appendicul.-ir skeleton. 372

deveIo])ment of. 4<>2 Aque«lu«t of Sylvius, development of,

29(> Arch, hyoid. 115

mandibular. 115

maxillary. 115

of aorta. d«*velo]»ment of, 107 Arched eolle(>ting tubule of kidney,

240 An'heiitenm. 52 Ar;lie>. aortic. 105

branehial. 114

mandibular. 1.35

visceral, 112 Arehibla«<t. Of! Arcuate fissure. 300. .307 Area, embryonal, 5S

glamlular, 275

opaca, 59

417

418

INDEX.

Area pellucida, 59

vasculosa, 59, 88, 150 Areas, iiasail, 145, 359 Areola, development of, 276 Areolar tissue, develo))tueiit of, 125 Arrectores piloruiii, 2b9 Arteria centralis retinae, development

of, ;«5 Arterial system, fetal, 165 Arteries, allantoic, 90, 164

umbilical, 103, 165

vitelline, 151 Artery, carotid, common, development of, 166 external, development of, 166 internal, development of, 166

innominate, development of, 167

middle siicral, development of, 166

pulmonary, development of, 168

subclavian, left, development of, 168 right, development of, 167

superior vesical, 182 Arytcno-cpiglottidean folds, 226 Arytcn»>id cartilages, development of, 22()

ridges, 22(> Ascending colon, formation of, 203

me.socolon, formation of, 203

root of lifth nerve, 224

root of vagus, 290 Aster. 45

Atlas, formation of, 381 Atresia of pupil, 3:tt. Atrial crescent, 157 Atrioventricular canal, 156

valves, ]56 Atrophic tubules of Wolffian bmlv,

23() Attract ion -sphere, 45 Auditory apparatus, development of. 3*15

meatus, external, formation of. 357

nerve, formation of. 321

nucleus, lateral accessorv. 321

]>it. 3I(» Auricle, (levelopuKMit of, 35R Auricles, division into right and left,

157 Auricular ap])eudages. 159

canal. 15<)

<;cptuni. 157 Auriculovcntricular a]»ertures. 161

valves. \i]'2 Axial tiher of spern«arozo<m. 20, 22

skeleton. 372

development of. 37'> Axis. (leveio]>ment of. 3>0 Axis-cylinder process. 2>1

IUui>kkn's primitive disk. 377, 379 Hartholiji. glands uf. '2til Basil ganuHia. .".(K]. ,3Hl

lamina. 2!>o Hasi-ocripital bojie. .390 Rasisphcnoid, 394

Belly-stalk, 85 Bifid uterus, 253

Bile-capillaries, formation of, 209 Bile-ducts, formation of, 209 Bladder, development of, 255 Blastema, Wolffian, 236 Blastodermic vesicle, mammalian, 50 stage of, 49

two- layered stage of, 52 Blastopore, 52 Blastula stage, 49 Blood, development of, 126, 147 Blood-islands. 148 Blood-lacunee, 97 Blood-platelets, 150 Blood-vessels, 150 "Blue baby," 158 Bodie^i, polar. 3.3, 34 Body of vertebra, formation of, 377 Body-cavity, 63, 6(>, 214 Body-wall, development of muscles of, 367

formation of, 79 Bony cochlea, development of, 352

labyrinth, development of, 351

semicircular canals, 351 Bowman, capsule of, 238, 240 Brain, development of, 286 Brain-case, 384 Brain-membranes, development ot,

302 Brain-vesicles, 287

derivatives of, 316 Bninchial arches, 114

development of, 369 Bran chiome res, 78 Bridge of nose, development of, 362 Broad ligament of uterus, 255 Brunner. glands of, 206 Bud, embryonic, 54 Bulbus arteriosus. 156

vestibuli. 259 Burdax'h. tract of. myelination of, 414 Bur>;i. omental. 20l,'21S

pharyngeal. 136 Bni-sal sacs, development of, 126

('ADrcors niembrjm<»s, 195 C'tecum, develo]mient of, 202, 203 Calcaravis. 308 Calcarine fissure, 303, 308 Callosoniarginal fissure, 309 ("'anal, anal, 257

atrioventricular, 156

auricular. 15()

hyaloid. 339

medullary. 70

neural. 70. 279. 281

jieurenteric. 74. 281

of anus. 197

«.f His. 145. 227

of Xuck. 255

of Stilling, 339 Canaliculi. hurrimal, development of, 345

INDEX.

419

Canalis reuniens. 349 Capsule of Bowman, 238, 240

of kidney, 241 Cardinal veins, 164 anterior, 189 posterior, 169 Carotid artery, common, development of, 166 external, development of, 166 internal, development of, 166

body, 325 Carpus, development of bones of, 405 Cartilage, formation of, 126

Meckel's, 115, 398

Reicbert's. 115 Cartilage-cells, 126 Cartilaginous capsule of cochlea, 352

cranium, 386

ear-capsule, 351

ribs, 382

sheath of spinal cord, 378

stiige of skeleton, 373 of trunk skeleton, 377

vertebral bodies, origin of, 379 processes, origin of, 379 Caudal aorta, 166 Cavity, amniotic, 54, 84

cleavage-, 50

pleuroperitoneal, ^

segmentation-, 50 Cell-cords, 150 Cell-mass, inner, 50

intermediate, 77, 232

outer, 50 Cells, sexual, 31

mesenchynjal, 66 Cementum of tooth, 137

development of, 141 Central canal of cord, formation of, 286

lobe, formation of, 305 Centrolecithal ova, 27 Centrosome, 45 Cephalic flexure, 112,288

ganglia, development of, 320 Ceratohyal, 402

Cerebellum, development of, 292 Cerebral fissures, development of, 302

vesicles, 287, 288 Ce numinous glands. 273 Cervical fistula, 116

flexure. 112

rib. 380. 383 Chalazsp, 29 Chambers of eye. 342 Chin ridge. 13.'> Chorda dorsjilis. 73 formation of. 373 stage of. 373 Chordte tendiiu'jp. 162 Chordul ei)ithelium, 374

plate, 74

region of primitive skull, 387 Choriata, 94 CIiorioc:;pillaris, 340

Chorion, 92

frondosum, 93

leve, 93

primitive, 92

true, 92 Choroid, coloboma of, 341

development of, 340

fissure, 306, 307

plexus, 308

plexuses of fourth ventricle, 291 Choroidal fissure, 330, 341 ChromafiSne cells, 325 Chromatin, 27

Chromosomes, reduction of, 23 Cicatricula, 28 Ciliary body, development of, 341

ganglion, 320

muscle, development of, 341

processes, development of. 333, 341 Circulation, allantoic, 90, 163

placental, 147

portal, 177

vitelline, formation of, 147 Claustrum, 303 Clavicle, development of, 404 Cleavage, kinds of, 47

of ovum, 45

partial discoidal, 48 peripheral, 48

total equal, 47 unequal. 47 Cleavage-cavity, 50 Cleavage-nucleus. 43 Cleavage-planes, 46 Cleft palate, formation of, 137

stem urn, 383 cause of, 82

uvula, fonnation of, 137 Clefts, visceral, 112 Climacteric, 38 Clitoris, development of, 259 Cloaca, 190, 196, 256 Cloacal depression. 197, 256 Closing membrane, 113, 117, 106 Coccygeal body, 325

curve, 112

vertebra?, ossification of, 382 Cochlea, bony, development of, .352 Cochlear duct, formation of, 347

ganglion. 321

nerve, .'154 Coplenteron, 52 Ccplom, 63. (>6, 214" Collateral fissure, 30,3. .308 Collecting tubules of kidney, 237 Coloboma of choroid, 341

of iris, 343 Colon, ascending, formation of, 203

descending, formaticm of, 201, 203

transverse, formation of, 203 Coluinnre carnete, 154 Commissures of brain, development of, 303

of cord, white, 285 Conarium, 298

420

INDEX.

Conariuiii. modificatious of, 298 Coue-visuul cells, 331 Congcuital iitresia of pupil. 33d <lia])brHKniatic heruia, 177 fecal fistula, 207 hernia, 249 umhilic4il hernia, 205 Colli vasoulosi, formation of, 246 Connective tissues, development of,

124 Constructive stage of menstrual cycle,

39 (.'oi>iila of hyoid hone, 3S9 Conicoid hone, 403

process of scapula, 40,3 Cord, spinal, <levelopment of, 2sl

umhiiical, 102 (^)^ds of cells. 147 Corium, development of. 2()8 Cornea, development of. 340 Cornicular tuberch'S. 2i<) Corona radiata. 2."). 31 Coronarv lipvment. 210 of liver. 221 sinus of heart, 172 valve. Kil Corpora alhicantia. 296 hijreniina. 29.1

cavernosa, formation of, 262 (juadrijjemina. '^9.") Corpu«; callosum, formation of, 309, 311 hemorrhaKicum. 37 luteum of prej^nancv. 37, 38 false, :W

of menstruation, 3S true. .38 spongiosum, formation of, 262 striatum, 303 Corpus.;le of Hassal. 230 Corti. or;ran of, 319 Costal process of vertebra, formation

of. 37«). 3'-2 Ci»tyl»Mlous of placejita. !»9 Coveriiii; l)ones, .3*»r> Cowper. ulaiuls <»f. 2<J3 Cranial capsulf, IJ*^!

nerve-fibers. dev«'lopmcnt of, 320 Cranium, cartilairiuous. 3s(> membranous, .385 ossL'ous. 3*^9 Crescent, atrial. l'>7 <'ricoid cartilai^e. 22»» ( 'rista' Mcusticu'. 350 (rossrd pvramidal tract. mvelinati<ui

of, 115 ('rum cerebri, develoi>ment of, 295 Crusta |)etrosa, 1 11 Cryptorcbism. 219

CrystjiUine lens. cl('veloi)ment of. .336 Cuueitorm tubi'rcle>. 22<» Cu-.]iioiis, endocardial. 15«) Cutis-plat'*. 77. 26S. :ij;.-> Cuvi«'r. duct of. Ml. 17(K 176 Cv-^tic dtirt. develo])ment of, 209 cVt.»bIast. 54

Daughter-cells, 22 Daughter-wreaths, 45 Decidua nienstrualis, 39, 95

of pregnancy, 96

reliexa, 9rocesses, 141

ridge, 1,37

shelf. 137 Den tale, 400 Dentate tissure, 30.3, 307 Dentinal fibers, 141

tubules, 141 IX'ntine, 137 Dermal bones, 385

navel, 82 Descending colon, formation of, 203 Descent of testicles. 218 Destructive stage of menstrual cycle,

39 Deutoplasm of hen's egg, 28

of ovum. 26 Develo])inent during eighth mouth,

during eighth week, 119

during fifth month. 121

during fifth week, 118

during ninth month, 122

during secoiul moiAh, 118

during seventh unrnth, 121

during sixth month, 121

during third month, 120

•luring tiiird week, 117

length of time necessary for, 18

tabulated chnmology of, 409

theories of. 17 Diaphragm. develo]>ment of, 177 Diaphragmatic hernia, congenital, 177

ligament. 248 Dieiicepbalon. 287 Digestive svstem. development of,

1.S5. 411-416 Diiritation of limb-buds, 407 Dipbyodont. 137 Dire<t cerebellar tra<*t. nivelination

of. 411 I)isc«>idal cleavage, partial, 48 Discus proiigerus, 31. 251 Disk, germinative. 28 Distal convoluted tubule of kidney,

210 Divrrticubi of primarv renal pelvis,

237 l)ors;il curve, 112

mesrnter.v. 1{H»

nerve-roots of s])inal ganglia, 318

pjincreas. 211 Double monster, origin of, .58

uterus, 2.53

Duct of Cnvier, 164, ITO, ITS luuouephric. £U of UutuKr. 251

ur Miuicr. 24a. air, 253, se&

i>f Batbkc. 246 <if SaotoriDi, 812 of Wirauug. as proDcphrlc, 233

8i'KllieuUl. 233

tlivrtiKlossal. 145. 227

tlivruld, B2T

vitelline, HO, fC, 1S»

Wuiffiau, 234 Ductus Arunlii. 180

arteriosus, 16H

commuDia chuledochus, formation of, 20!)

vndol Till p till lie us, 347

veiuhiUs. lli:i, l-u Duodi'Dum. furtuution of, 217

Ear. eitemiLl, development of, 355. 358 iiiliTiiiil. Ji'velopnient of, 316 iiiiildl, ilcvelopmonC of. 35G

l;.riin-iiU\ cartiUginomi, 351

Ectudemi, 'fi

Egg. iihiinate origin of, 31 E^Wuiuns. 31. iHQ E«K-^'>v«lapeB, 85 Egg-plasm. 26 EKg-lubi», priniary, 31 EigbCh mouth, ilovelopnicat during. 123.416

pair cranial nervea, developmeat of, 323

week 'development during, llil, 412 Ellaculiitarjr iluct. furmntlan of, 347 Elaatii: tissue, formatian of. ]^ Elevontb paircrnnial ii.TTes. 324 EmbpddiiigofDX'iini. M Embryo, diScreiitiuIion of. ffi>

of elffht and B half weclu, 121

of lin«ctitlj riBT, Its

c.frtli weeks, I'lP

of hirtwnthdnr. lOS

of three weekx. 112

of tirenlv-eight days, 118

sejtnii-iiUlion nf body of, 78

slSKi-of. in. 107 Eiubryolijay de fined, 17 Enibryouiil area, .>8

Embryonic bud. I>4 cn-KCcnl. -")!)

Eutl-kuob i.f lipunUBtozooii, 21. 22 EriducardiaU'nslii..iis lat* Eudoutrdiiim. 1>4 Endochoudral Uiuus, 3»5 Eudolyniph. aoA Cndoskeloton, 3T^ KiLili'ih'liuiii. I'liniintluu of. 66, 126 Eiid-piPd- i,r swrmaf

] I ' . i'.ii.. ; \.:\:r of loammaliau

l>l:if ttnlrtmjo vesicle, QO Ependyma. 310 Epcndymal celU, 392. £83

E|.ihyttl, 402

Epiotic center uf usdtlcation, 392 Epitbeliul lHHlie«. ^9 Epithelium, terminal. '29, 31, 244 Epitriebium, 2U0

Efn^'phoroiuVVl Erythruhlustii. 14^ Erythrocytes, 119 Elhniold bune. iwifiuition of, 305

cribriform plate of. 388 Ethmoidal sinuB. dDvelopmcnt of, 3^1 Euatnchian tube, development of, fJW fominli'iuof, 19-1

Vhtve

Exoccipitals. 300

Eioskclelon, 372

ExBtmphy iif lihiddor, cause of, fi3

Eitcrniil uaditory meatus, formation

of, 31.-),

jimiitiils, ^mu!.^ 250, 266

.uiiic. an. iW

Eyp. devoli.pment of. I'lH. 326 Evclashoi. duvclopment of, 344 Eyvlid, third. 344 Eyelids, development of. 343 primitive, 134

Fack, derclopmcnt of, 117, 130 Fai'ial KHKRllon, 321 Falciform ligament of liver, formation of. 210 lobe, 309. 313 Fnllopian tube«, development of, 253

1. SI I Fall cerebri, 303

422

INDEX.

Fecal fistula, congenital, 207 Female external genitals, 259, 266

internal genital organs, 249

pronucleus, 34

sexual system, 266 Fertilization, 41

artificial, 44

external, 42

internal, 42 Fetal arterial system, 165

membranes at birth, 104

vascular system, final stage of, 181

venous system, 169 Fetus, length of, at term, 122

stage of, 20, 118

weight of, at term, 122 Fiber-tracts of cord, development of, 285 myelination of. 414, 415 Fibrillee of muscle, formation of, 366 Fibrous tunic of eye, development of,

339 Fifth brain-vesicle, metamorphosis of, 289

month, development during, 121, 414

pair cranial nerves, development of, 323

ventricle. 312

week, development during, 411 Fimbria, 309

Fingers, development of, 407 First pair cmnial nerves, development of, 323

week, development during, 409 Fissure, arcuate, 306, 307

calcarine, 303, 30H

calloso-niarginal, 309

choroid. :W), 307

choroidal, 330

collateral, 303, 308

dentutc. 303, 307

great transverse. 304, 308

hippocampal. 307

of choroid ]>loxus, 3^)7

of Kohmdo. 30,S

of Sylvius, 303, 301

purieto-occii»ital. 308 Fissures, cerebral, development of, ,^02. 303

median, of cord. 285 Fistula, congenital fecal, 207

M Ml hi Heal urinary, 25<j Flexure, cephalic. 112. 288

nuchal, 289

pontal, 2.^9 Floor-i)late, 2.'-l. 282 Fold, ])h'uropericardial, 176 Folds, medullary. 72 Folliflo. (Jraafian. 2J»

of tooth, 1 11 ForauHMi cR'cuni. 115, 227

eomnunu' anterius. 30()

of Monro. 301. 3<M)

of V/inslow, 221

Foramen ovale, 157

thyroideum, 226 Fore-brain, 286, 302

secondary, 287

vesicle, 73

metamorphosis of, 302 Foregut, 81 Formative yolk, 26 . Fornix, formation of, 309, 310 Fossa of Svlvius, 304

oral, 192

ovalis, 158 Fourth month, development during, 120, 414

pair cranial nerves, development of, 323

ventricle, 291 development of, 290, 294

w^eek, development during, 410 Fretum Halleri. 156 Frontal bone, ossification of, 396

lobe, 306

sinuses, development of, 361 Funiculus solitarius, 290 Furcula, 225

Gall-bladder, development of, 209 Ganglia, cephalic, 320

spinal, 317 Gangliated cord of the sympathetic,

325 Ganglion, acoustic. 321 acusticofacial, .'^21 cephalic, fourth, 321

third, 321 ciliary. 320 cochlear. 321 facial. 321 Gasserian, 320 intercarotid, 325 Luschka's. 325 ophthalmic, 320 s]>irale, 350 trigeminal, 321 vestibular, 351 Ganglion -cell laver, development of,

333 Gartner, duct of, 254 Gasserian ganglion, 320 Gastrjil mesoderm, 63 Gastrohepatie omentum, 209, 220

formation of, 205 Gastrosplenic omentum, 214 (rastrula, 52

stage, 52 Generative organs, external, development of. 258 internal, development of, 243 Genital cord. 243 eminence. 259 in male, 261 folds. 259

in female. 259 in male. 262 gland, indifierent, 265

INDEX.

423

Genital groove, 258

ridge, 243, 258 in female, 259

ridges, 31 Genito-urinary system, development

of, 232, 409-416 Germ-cells, 224 Grcrm-disk, 27 Germ-layers, 52

derivatives of, 67 Germinal epithelium, 29, 31, 244

sj)ot, 25, 27

vesicle, 25, 27 Grerraiuative disk, 28 Giral<les, organ of. 247 Glands of alimentary tract, formation of, 206

of Bartholin, 261

of Brunner, development of, 206

of Cowper, development of, 263

of intestine, development of, 206

of Lieberkiibn, development of, 206

of Moll, 273

of stomach, development of, 206 Glandular area, 275

hy]K>spudias, 262 Glaus clitoridis. formation of. 259

penis, formation of, 259, 262 Glasorian fissure, 393, 399 Globular processes, 118, 132, 360 Glomerulus of kidney, 233, 238, 240 Glomus caroticus, 325 Goll, tract of, myelinatiou of, 414 Graafian follicle, 29 development of, 251 formation of new, 252 Gray matter of bniin, formation of, 303 of medulla, development of, 224 Great omentum, formation of, 204,

220 Groove, dental. 140

lacrimal, 119, 132

medullary, 71

naso-optic, 345

primitive, 60

pulmonary, 22.S

transverse, crescentic, 398 Gubernaculum testis, 248 Gum, development of, 136 Gut, postanal, 196 Gut-tract, 80. HI. 186, 188 Gyrus fornicatus, 315

uncinatus, 315

Hair, development of, 271 Hair-bulb. 271

development of, 272 Hair-follicle, 271.

development of. 272, 273 Hair-germs. 272

Hard palate, development of, 397 Hare-lip, 134, 397 Hassal, corpuscles of, 230

Head, muscles of, development of, 367

of epididymis, 246

of spermatozoon, 20, 22 Head-fold, 80

of amnion, 80, 83 Head -gut, 188 Head-kidney, 232 Head-process of primitive streak. 62,

70 Head-segments, 364 Head-skeleton, development of, 384 Heart, development of, 152

lymph-, 128 posterior, 128

metamorphosis of single into double, 156

valves, development of, 161 Helix, formation of, 358 Hemal arch, formation of, 376 Henle's loop, 240 Hen's egg, description of, 27 Hensen's node, 62 Hepatic cylinders, 209

vein, development of, 181 Hermaphroditism, 263, 2()7 Hernia, congenital, 249

umbilical, 205 Highmore, antrum of, development

of, 361 Hilum folliculi, 31 Hind-brain. 28(J. 292

secondary, 287

vesicle, 73, 292 Hindgut. 81, 1H8 HipiKicanipal fissure, 307 Hippocampus migor, 307

minor. 308 His, canal of. 145, 227 Holoblastic ova, 47 Homogeneous twins, origin of, 59 Homologies of the sexual system, 263 Hyaloid arterj*, formation of, 339

canal, 339

membrane, formation of, 339 Hydatid of Morgagni, 247

8i»s8ile, 247

stalked, 247

unstalked, 247 Hydramnios, 86 Hymen, formation of, 261 Hyoglossus. origin of. 370 Hyoid arch, anterior, 389 l)osterior, 3'^9

arches. 115

bar. 3H9

bono, development of. 389, 401 Hyoidean apparatus, 401 Hyonmndibular cleft, 115 Hypobhist. 5'^ Hypochordal brace, 376 Hypophysis, ,'^(X>

formation of. I'i5 Hypospailias, 262

glandular, 262

424

lyDEX.

Iliac segment of pelvic girdle, 404 vein, left cuiumun, development of,

I m perforata anus, 197 luipresisious, maternal, 120 Ini'us, development of, 388, 399 Indifferent genital gland, 265

si'xual gland, 244 Inferior medullary velum, 292

peduncles of brain, 290 Infundibula of lungs, development of,

225 Infundibulum of brain. 2i>6, 300 Inguinal ligament, 248

in female, 254 Inner cell-mass, 50 Innominate artery, development of,

liu Inter-brain, 287, 296

vesicle, metamorphosis of, 296 Intercarotid ganglion, 325 Intermaxillary bones, formation of,

l.iei, 397 Intermedial cell-mass, 77, 232, 365 Internal ear, development of, 346

fertilization, 42

lateral ligament of lower jaw, 400

limiting membrane of spinal cord, 3v'^3 Interpallial fissure, 302 Interrenal organ, 242 Intervertebral disks, 377, 379

ligament, development of, 377, 379 Intervillous spaces. 97, 102 Inte.stinal canal, formation of, 79

glands, development of, 20*>

mesentery, 216

mucosa, formation of. 189

villi, formation of, 206

]>ortals, 81. 186 Intestine, small, development of, 202,

205 Intestino-bodv cavitv, 52 Intumescentia ganglioformis. 351 Involuntarv muscle, development of,

371 Iris, coloboma of. 343

development of, 341 Ischiatic rod. 404 Island of Keil, :J05

.Tacobson's organ, development of.

3()1 Jaw. upper, development of, 134 Jaw-arch. 115 .lellv of Wharton, 103 Joint-cavities, development of, 128 Jugular vein, primitive. 1({4, 169 transverse. 172

Kidney, development of, 232

Labia majora, 260

minora, formation of, 259 Labyrinth, bony, development of, 351

Labyrinth, membranous, development of, 346 Lacrimal bones, ossification of, 396

canal iculi, 345

caruncle, 344

duct, development of, 344

gland, development of, 344

groove, 119, l.'i2

sac, development of, 345 Lamina cinerea, 296, 299

quadrigemina. 295

spinilis, bony, development of, 354

terminal is, 309 Langhans' laver, 97 , lanugo, 121, 273 I I^iryux, development of, 225 I Uitebra, 29 Lateral cartilage of nose, 395

folds of amnion, 80

frontal processes, 118, 132, 134 in formation of nose, 146

ligaments of liver, 210

nasal process, 344, 360

plate of mesoderm, 65

plate of somite, 63

ventricle, development of, 303 length of fetus at term, 122 Lens, crystalline, development of,

336 Lens-area, 328

Lens-capsule, development of, 337 Lens-pit, 336

Lens- vesicle, 110, 134, 328, 336 Lenticular zone of optic cup, 333 Lesser omentum. 220 formation of, 205 Leukocytes, 149 Levator palati, origin of, 370 Lids, union of edges of, 343 LieberkiJhn, glands of, 206 Ligament of ovary, 255 Ligamenta intermuscularia, 365, 375

subflava, 379 Ligaments of liver, formation of, 209 Ligamcntum venosum Arantii, 184 Liguhr. 2\yZ Limb-buds, 119. 406 Limbic lobe, 309, 313 Limb-muscles, development of. 370 Limbs, bones of. development of, 405

development of, 406, 409-416

position of, 407 Limiting membrane, intier, formation of, 3:ji outer, formation of, 331 Lin in, 27 Lip ridge. 135

upper, development of, 136 Liquor amnii, 85. 86 function of. 86

n.lliculi, 31. 251

of Morgagni. 337 Liver, development of, 207

first rudiment of, 198

ligaments of, formation of, 209

INDEX.

425

Liver-ridge, 175, 208 Iiobes of liver, 208 Lobule of ear, development of, 358 Longitudiual liber-tracts of medulla, 2«0

fissure of brain, 302 Loop of Henle, 240 Lower jaw, ossification of, 398 Lumbar rib, .'i83

vertebne, ossification of, 381 Lungs, development of, 223 Luschka's ganglion, 325 Lymph, formation of, 126 Lymph-clefts, development of, 128 Lymph-hearts, 128

posterior, 128 Lymph-sacs, development of, 127 Lymph-spaces, development of, 127 Lymphatic system, development of, 127

vessels, development of, 128 Lymphoid follicles of tonsil, 195

tissue, development of, 1'^

Macula lutea, formation of, 333 Maculse acustictc. development of, 350 Malar bone, ossification of, 396 Male external genitals, 261, 267

internal genital organs, 245

pronucleus, 42

sexual system, 245, 266 Malleus, development of, 388, 399 Malpighian corpuscle, development of, 213, 238 primitive, 236 Mammalia deciduata, 99

indeciduata, 99 Mammals, blastula of, 49 Mammary gland, development of, 274 Mandible, ossification of, 398 Mandibular arch, 115, 135, 386 Mantle layer, 284 Marginal sinus, 102

velum of spinal cord, 283, 284

zone of optic cup, 334 Marshall, vestigial fold of, 172 Maternal impressions, 120 Maturation of ovum. 32 Maxilla, superior, ossification of, 397 Maxillary arch, 115

process, 135, 386 Meatus, external auditory, 357

urinarius, male, 262 Meckel's cartilage, 115, 388, 398

diverticulum, formation of, 207 Meconium, 122 Median fissures of cord, 285

lobe of cerebellum, 292 Medulla oblongata, development of,

289 Medullarv canal, 70

cords, 246. 252

folds, 72. 279

farrow, 71

groove, 71

Medullary plate, 70, 279 tube, 279

velum, anterior, 294 inferior, 292, 294 Meibomian glands, development of,

344 Membrana adamantiua, 139 basilaris of cochlea, formation of,

eboris, 141

granulosa, 31 formation of, 251

prseformativa, 141 Membrane, anal, 193

closing, 113, 117, 194

nuclear, 27

of Xasmyth, 140

of Reissner, 355

pharyngeal, 117, 131, 188, 192

vitelline, 25, 26

tympanic, 194, 357 Membranes, caducous, 95

deciduous, 95 Membranous bones, 385

cranium, 385

labyrinth, development of, 346

ribs, 382

stage of skeleton, 373 of trunk, 374 Menopause, 38 Menstrual cycle, 39 Menstruation, 38

relation of, to ovulation and conception, 40 Meroblastif ova, 48 Mesencephalon, 286, 294 Mesenchymal cells, 66

muscle, 371 Mesenchyme, 66 Mesenteric artery, superior, 152

vein, superior, 181 Mesenteries, 190 Mesentery, intestinal, 216

ventnil, 204 development of, 220 Mesoblast, 62

Mesoblastic somites, 65, 75 Mesooardium anterius, 153, 174

posterius, 153, 174 Mesocolon, ascending, production of, 203

formation of, 203 Mesonephrogenic tissue, 239 Metanephrogenic tissue, 239 Mesoderm, 62

derivatives of, 68

gastral, 63

paraxial, 65

peristomal. 63

somatic, 66

splanchnic, 66

structures developed from, 12Aet8eq. Mesodermal vitreous, 338 Mesogastrium, 204, 216 Mesonephric duct, 235

426

JSDEX.

Mesonephros, 234, 264 Mesorchiuu, 24b, 255 Mcsothelium, (Hi, 126 McHovarium, 24>::^

Metacar|>al l>oneg, development, 405 MetamorphoHiH of single into double

heart, l.VJ MctanephroH, 237, 265 Metatarsal boneif, development, 405 Meteiiceplialon, 287, 292 Me topic suture, 396 Metopism, 396 Micropyle. 25, 42 Mid-brain, 2»6, 294

prominence of, 288

vesicle, 73. 294 Mid-gut, las Middle ear, development of, 194, 355

piece of Kpermatozoon, 20, 22

plate. 77, 232, 3<I5

sacral artery, development of, 166

tunic of eye, development of, 339 Milk-lines. 275 Mi Ik -ridges, 275

Mo<liolus of cochlea, development, 354 Moll, glands of. 273 Monorchism, 249 Monro, foramen of, .'$01, 306 Mons v<?neris, formation of, 259 Morgagni, hvdatid of, 247

liquor of, 337 Morula, 45 Mother-cells, 22

Motor nerve-fibers, development, 319 Mouth, development of, 134, 192 Mucous tissue, formation of, 125 Mulberry -mass, 45 Miiller, duct of, 243, 247, 253, 265 Miillcr's lib«;rs, :W1 Muscle, involuntary, development, 371

Vdluntary, development of, 363 Muscle-plate, 78, .365

metamorphosis of, .366 Muscles, bnmchial, development of, .3()9

of extremities, development of, 370

of trunk, development of, .363 Muscular coat of inti'stines, formation of, 205

system, development of, 363, 409416 Musculi papillares. 163

pectinati. 154 Myeleiucphalon. 287. 289 Myelin. <leposit of. 319 Mv<K-(el, .365 Myotonu", 77, 365

N'ml-rki). 271 Nail-phite. 270 Nails, development of, 270

of toes, 271 Nail-welt. 271 Nares, anterior, formation of, 146

develo])ment of, 3<)0

Nasal areas, 145, 359 bones, ossification of, 396 capsule, 388

cavities, development of, 361 pits, 118, 132, 145,360 process, 132. 360 lateral, 344, 360 Nasmych, membrane of, 140 Nasofrontal process, 115, 118, 132, 134, 360, 386 in development of nose, 145 Naso-optic furrow, 132, 134 in formation of nose, 146 groove, 345 Nephridial funnels, 233 Nephrogenic tissue, 236 Nephrostomata, 2«33 Nephrotome. 77, 234, 264, 365 Nerve-cells, formation of, 2c»2

of cord, formation of, 284 Nerve-corpuscles of neurilemma, 319 Nerve-fiber, envelopes of, formation of, 319 layer, development of, 333 Nerve-fil>ers, cranial, development of, 320 motor, development of, 319 sensory, development of, 317 Nerve-trunk, spinal, development of,

319 Nervous system, development of, 278, 409-416 peripheral, development of, 316 sympathetic, development of, 324 Neural canal, 70, 279, 281 crest, si'gmentation of, 318 crests, 318 process of vertebra, formation of,

376 tube, 279 Neurenteric canal, 74, 281 Neurilemma, formation of, 319 Neurit, 278, 284 Neuroblasts, 282, 284 Neuro-epithelium of retina, development of. .3.33 Neuroglia. 2S2.283

layer, 284 Neurons. 278

Nictitating membrane. 344 Ninth month, development during, 122. 416 pair cranial nerves, development of,

.324 week, development during, 413 Nipple, development of, 276 Node. Hensen's. (>2 Normoblasts. 149 Nose, development of. 145, 358 Nott>chord. 73

Notochordal stage of skeleton, 373 Nuchal flexure, 2S9 Nuck. canal of. 255 Nuclear.juice. 27 layer of retina, outer, 332

INDEX.

427

Nuclear membrane, 27

ttpiiidle, 45 Nucleus amygdalae, 303

cleavage-, 43

of uvuiii, 27

scgmeutation-, 43 Nutritive yolk, 26 Nymphffi, formation of, 259

Obex, 292

Occipital bone, ossification of, 390

lobe. 30<) Odontoblasts, 141 Odontoid process, development of,

3m Olfactory bulb, 314

epithelium, 359, 362

lobe, 314

nerve-fibers, 362

plates, 132, 145, 358

tract, 314 Omental bursa, 204, 218 Omentum, gastrohepatic, 209, 220 formation of, 205

gastrospleuic. 214

great, formation of, 204, 220

lesser, 220 formation of, 205

phrenicosplenic, 214 Omphalomesenteric veins, 151 Ontogeny, 17 Oocytes, 32 Oogenesis, 29 Oogouia. 32

Ophthalmic ganglion, 320 Opisthotic center of ossification, 392 Optic cup, 328

secondary, 330

lobes, formation of, 295

nerve, development of, 335

thalami, 29fj

vesicle, 287. 327 Ora serrata. 'Xil Oral cavity, development of, 192

f ( JSSft 1 ' f"^

pit. lOH. 117, 131. 135, 192

plate, l.SO, 134, 192 Orbitonasal center, 397 Orbitosphenoids, 394 Organ of Corti. 349

of Giraldts, 247

of .Tacobson, development of, 361

of Kosenniuller, 254 Ors^ans of Ziirkerkandl. 325 OsstMMis cnmiuni, 3H9

stag«» of trunk skeleton. 379

tissue, formation of, 126 r)ssirlcs of ear. d(*velopment of, 356 Ossification of ribs, 383

of skull. :}H9

of st<Tnuni. '.V<\

of vcrtcbne.'JrsO Ostium int(?rv«'ntrieulare, 158 Otir v<'sirlc. 109, :i46 Otocyst, 34()

Outer cell-mass, 50 Ova, alecithal, 26

centrolecithal, 27

classification of, 26

formation of, 29

holoblastic, 47

meroblastic, 48

primitive, 31, 245, 250

telolecithal, 26 Ovaries, change of position of, 254 Ovary, development of, 249 Oviducts, development of, 253 Ovists, 18 Ovulation, 36

relation of, to menstruation, 40 Ovum, 24, 251

embedding of, 95, 96

maturation of, 32

rii)ening of, 32

segmentation of, 45

stage of. 19, 106

Palate bone, ossification of, 396

formation of. 136

process, development of, 397 Palate-shelves, 3<)0 Palatoglossus, origin of, 370 Palatopharyngeus. origiu of, 370 Palpebral fasciae, 344

fissure, 343 Pancreas, development of, 211

dorsal, 211

first rudiment of, 199

ventral, 211 Pancreatic duct, development of, 211 Pander's nucleus, 28 Panniculus adiposus, 269 Papilla? of tongue, formation of, 145 Parablast, (Hi

Paraehoi-dal cartilages. 387 Paratlidymis, 247 Parathyroid bcnlies. 229 Paraxial nies<Mlerm. 65 Parietal bones, ossification of, 396

elevation, 288

eye, 299

fomnien, 2JW

layer of pleura. 177

lobe, 3(KJ

zone. 76 Parieto-oocipkal fissure, 308 Pan»oj)horon. 254 Parovariunj. 254 Pars ciliaris retime. 334

i n te rm ed i a 1 i s, 259

iridiea retina?. 334

menibranacea septi, 159

optica retinje. 333 Parthenogenetic <'ggs. 34 Patuh)us foramen ovale, 157 Pectoral girdle, development of, 403 Pelvic girdle. 404 Pelvis of kidney, primary, 237 Penis, development of, 259 Perforated lamina, anterior, 315

428

ISDEX.

Perforated space, posterior, 295 Pericar<lial aivity, 175 Pericardium, development of, 174 Perilymph, 352, 355 Perilymphatic space, 352 Perineal bwly, 197 Perineum, formation of, 197 Perionyx, 271 Periotic bone, 392 Perijiheral cleavage, 48

nervous system, 316 Peristomal mesoderm, 63 Peritoneal cavity, 215 Peritoneum, development of, 214

visceral layer of, 189 Perivitelline space, 25 Permanent kidney, 237

teeth, development of, 141 eruption of, 143 Petromastoid bone, 392 Petrotympanic fissure, 399 Pfliiger's egg-tubes, 250 Phteochrome bodies, 242

cells, 242 Phalanges, development of, 405 Pharyngeal bursa, 136

constrictors, origin of, 370

membrane, 117, 131, 188, 192 in formation of mouth, 135

pouches, 113, 188, 193 Pharynx, 193

Phrenicosplenic omentum, 214 Phylogeny, 17 Pial processes, 283 Piiimcnt-lavcr of retina, 331 Pillars of Uskow, 177 Pineal body. 296

or gland, 2f)7, 298

eye, 299 Pit, auditory, 316

oral, 1 OS 117 Pits, nasi I, 360 Pituitary body, 300

formation of, 135 Placenta, 98

at term, 101

discoidoa, 99

pnevia. 102

zonaria, 99 Placental sinuses, 100

spaces. 102

system of blood-vessels, 164 PlaVentoblast. 51 Plaues of cleavage, 46 Plantar born, 270 Plasiuodobljist, 54 Plate, chordal. 71

medullary. 70

vertebral, 65 Pleura, parietal layer of, 177

visceral layer of, 177 Pleune. develoi)ment of, 174, 175. 226 Pleural sjics, formation of, 174, 175 P]euroi)eri('ardial fold, 176 Pleuroperitoneal cavity, 66, 215

Plica semilunaris, 344 Pocket of Rathke, 301 Polar bodies, 33, 34

striation, 45 Polarity of egg, 27 Pole-corpuscles, 3ii Polyphyo<lont, 137 Polyspermia, 42 Pontal flexure, 289 Pons, formation of, 292 Portal circulation, 170, 177

vein, development of, 181

venous system, 170, 177 Postanal gut, 196 Postbranchial bodies, 229 Posterior chamber of eye, 342, 343

nare«, development of, 360 Post-limbic sulcus, 309 Postsphenoid, 393 Preformation theory, 18 Prehepaticus, 175, 208 Prehyoid gland, 227 Premaxilla, 397 Prepuce, formation of, 262 Prespbenoid, 394

Primary collecting tubules of kidney^ 239

egg- tubes. 31

renal pelvis, 237 Primitive aorta, 151, 165

chorion, 92

disk, 377, 399

enamel -germ, 138

eyelids, 134

groove, 60

heart- valves. 161

jugular veins, 161, 169

Malpighian corpuscle, 236

nails, 270

ova. 31, 2-15, 250

segment i)late, 65

segments. 65. 75

sexual e«dls, 245

streak. 59

vertebral bow, 376

vitreous. 33S Primordial bones, ,*^5 Proamnion. i\\ Process, latenil frontal, 118, 132, 134

nasal. 132, 3()0

nasofrontal. 115, 118, 132, 134, 360 in formation (jf nose, 145 Processes, dental, 141

globular. 118, 132, 360 nasal, 360

maxillary, 135

of vertebra, development of, 376 Processus vnginalis, 249 Proeborion, 50, 92 Proctodeum. 196 Pronei)bric duet, 233 Pronephros, 232. 264 Pronucleus, female, 34

male, 42 Pro-otic center of ossification, 392^

INDEX.

429

Prosoncephalon, 28(5 Prostate kI&"^* formation of, 257 Prostatic urethra, formatiou of, 257 Protoplasmic processes, 264 Prottivertebru, 6.3

Proximal convoluted tubule of kidney, 240 Pterygoid plate, internal, development of, 394 Pubic rod, 404

Pulmonary alveoli, development, 225 artery, development of, 159, \G6 diverticulum, 223 grtK>ve, 223 Pulp of spleen, development of, 213

of teeth, 137 Pupil, :VM) congenital atresia of, 333 development of. ;W2 Pvnimiilul process of thyroid gland, 22H tracts, anterior develojmient of, 290 crossed, of cord, mvelination of, 415

Ramus communicans. 325 Riithke's piK-ket, 13<>, 193, 301 Kiiuber's layer, 50 Ki'ceptaciilu chyli. 12S Keceptive i)rominence, 42 Kecessus labyrinthi, kWI

vestibuli. .'M7 Kectuui, 197

Ri'current laryngeal nerves, 1(38 Ke<luctiou of chromosomes, 23 Reduction-division, 23 Reichert's cartilage, 115, 389, 401 Reil, island of, 305 Reissner, membrane of, 355 Renal vein, left, 173

vesicles, 210 RepHMliiction, theories of, 17 Re>pinitorv svstem. development of,

222,'40i>-41() Restiform bodies, develoimieut of, 290 Rete mucosum. 270

testis, formation of. 21() Retina, development of, \^Z6 Rhinencephalon, 314 Rhombencephalon, 2H() Rhomboidal fossji, 2i)l i:ib, cervical, :W0. 3^3

lumbar, 38.3

thirteenth, 383 Rib-j, development of, 382 Ri«lne, genital, 213

terminal, 58 Ring lobf, formation of, 304 Ri|M'Mlug of ovum. 32 Roil-and-cone laver, formation of, 332 Rod-visual cells,\'{;n R(»lando, fissure of. 30^ R(M)f-plate, 2'^1, 2>^2 Rotation of stomsu'h, 203, 217 Round ligament of liver, Irtl

Round ligament of liver, formation of, 210 of uterus, 248, 255

Saccule, development of, 349 Saccus endoh'mphaticus, 347 Sacral vertebrse, ossitication of, 381 Sacrum, formation of, 381 Salivary glands, development of, 143 Santorini, duct of. 212 Sauropsida, blastula of. 51 Scala media of cochlea, development of, 347

tympani, development of, 355

vestibuli. development of, 355 Scapula, development of, 403 Schwann, white substance of, 319

deposit of, upon libers of tract of conl, 411, 415 Si'lerotome, 77, 365, 375 Scrotum, development <tf, 263 Selniceous glands, development of, 274 Second month, development in. lis

IMiir cranial nerves, development of, 323

week, development during, 40i) Secondary hair, 273

optic cup, ;J30 Secreting tubules of kidney, 237, 239 Segmental duct, 233 Segmentation of body of embryo. 78

of ovum, 45 S(;gmen tat ion -cavity, 50 Segmentation-nucleus, 43 Semicircular canals, bony, 351

<levelopment of. ,'i4.S SiMuilunar valves, development of. KC Sc>minal ampulhe, 21<)

vesicle, format i(m of. 247 Seminiferous tubules, formation, 246 Sense (ngans, development of, 32t),

409-416 Sensory epithelium of retina, 331

nerve-libers, development of, .317. 31s Septa placenta*. 102 Si'ptal cartilage of nose. 395 Septum, aortic, 159

auricular. 157

intermedium. 1.56

luciduni. fornuition of. 312

primum. 157

secundum, 157

spurlum, 1.59

transvcrsum. 164, 175 SeroMi. si

Si'rous membranes, development, 126 Sertoli's columns. 21. 246 Sessile hydatl«l. 217 Seventh month, development during. 121, 415

pair cranial nerves, development of, 32,3

week, development during, 412 Si^xual cells. 31

430

INDEX.

Sexual cells, primitive, 245

cords, 31, 245 female, 250

gland, inditl'erent, 244

system, female. 24f», 2t>6 homologies of, 2t).'i inditferent type, 243 male, 245, 2m SJiell of lien's egg. 29 She 11 -membrane, 21) Shoulder girdle, development of, 403 Sinus, annular, 179

pcK'ularis, 247, 257

pneeervic^ lis, 110

reuuiens, 159

terminal is, 150

urogenital, UK), 256

venosus, 159, 169 Sixth month, development during, 121, 415

pair cranial nerves, develo])mentof, 32.3

week, development during, 119. 411 Skelelogenous sheath of chorda dorsal is, 375

tissues, 77 Skeleton, appendicular, 373

development of, 402, 409-416

axial. 373

development of, .372

of head, development of. 384

of trunk, cartilaginous stage, 377 chonlal stage of. 373 development <»f. 373 membranous stage of, 374

visceral. 3>4 Skin, ajipeiulujjes of, 270

development of. 2(»S. 409-116 Small intestine. develo]unent of, 205 Snu'gnia enibryonum. 270 Somatic mesmlcrni. 6(5 Somatopleure, 6»l, 1>6 Sornitrs. 6.3, 75

mesoblastic, 65. 75 Space, peri vitelline. 25 Spaces, intervillous. 97. 102 Spermatie cord, 249

veins. 173 Spermatids. 22 Spermatoblasts. 2*? Sperniatogenesi>. 'Jl Spermatoyenic eell>. 21 SpiTUiatocytes. primary, 22

S4'0niid:irv. 22 SjM'rmato<;(»Tiia. 22 Spernialozoon. tjo

power of loeoiMotion of. 21

vitJility of. -Jl Splu'uoid b«t!ie. os>iri(at ion <»f. 39."» S]>heTioi«lal sinus, development of. ."',61 Spinal eonl. <lev<]«ipmeiil of. •^•'1 Spinous p^oee«^> of vertebra, develop nn'Tit of. .'Jsn Splaiiebuie nn>.od<'rni, 66 Splanebnopleure. i\i\, INJ

Spleen, development of, 212 Spongioblasts, 282, 283 Spot, germinal, 27 Sprouts, vessel, 150 Squamozygomatic bone, 391 Stage of embryo, 19, 107

of fetus, 20, 118

of ovum, 19, 106

of quiescence of menstrual cycle, 40

of rei>air of menstrual cycle. 40 Stalked hydatid, 247 Stapes, development of, 389 Stem-zone, 75 Sternum, cleft, 383

development of, 383 Stigma. 31

Stilling, canal of, 3:i9 Stomach, development of, 203

first rudiment of, 198

glands of, development of, 206

rotation of, 203, 217 StomodsBum, 131, 192 Straight collecting tubules of kidney,

239 Stratum Malpighii, 270 Streak, j)rimitive, 59 Striated muscles, development of, 303 Stroma-laver of choroid, development

of, 340 Styloglossus, origin of, 370 Stylohyal, 402

cartilage, 393 Stylohyoid ligament, 389 Styloid process of hyoid. 389

temponil, development of, 393 Stylopharyngeus, origin of, 370 Subclavian arterv, left, development of, 1(38 right, development of, 167 Submucosaof intestines, formation of,

205 Substance-islands, 147 Subzonal layer of mammalian blasto <lerniic vesicle, 50 Sulcus interventricularis, 1.58

of corpus callosum. ;i07

terniinalis, 160 Suju'rior maxilla, ossification of, 397 Suprahyoid gland, 227 Supra-<M'(*ipital bone, 390 Su]M"aperi('ardial bmlies, 228 Suprarenal bo<lies, development of,

241. 265 Suspensorv ligament of liver, fonna tioil of. 210 Sustentacular cells of seminiferous

tubule. 21 Suture, amniotie, KJ Sweat-glands, development of, 273 Sylvius. a<iueduet of, 2fW>

' ti-sure of. ;}n:;, :ioj

l'oss;» <)f. 1504 Syujpathetie nervous sy.stem, 324 Syncytium. 93, 97 Synovial sacs>. dev<'lopment of, 126

/

INDEX.

431

Tail of spermatozoon, 20, 22

Tail-fold, «0

Tarsal ligameuts, 344

plates, 344 Tarsus, development of bones of, 405 Teeth, development of, 137

permanent, development of, 141 eruption of, 143

temporary, development of, 137 eruption of, 142 Tela choroidea, 297 Telencephalon. 287, 302 Telolecithal ova, 26 Temporal bone, ossification of, 390

lobe, formation of, 304 Temporary teeth, development of, 137

eruption of, 142 Temporomaxillary articulation, 400 Tendon, development of, 125 Tendon-sheaths, development of, 128 Tenth pair cranial nerves, development of, 324 Terminal filament of spermatozoon, 20,21

ridge, 58 Testicle, development of, 245

descent of, 248 Thalamencephalon, 287, 296 Thebesius, valve of, 161 Theca foUiculi, 29 Thecal sacs, development of, 126 Theory of evolution, 17

of unfolding, 17 Third eyelid, 344

month, development in, 120, 413

pair cranial nerves, development of, 32:i

ventricle, formation of, 296

week, development during, 410 Thirteenth rib, 383 Thoracic prolongations of abdominal

cavity, 175 Throat-pockets, 113, 188, 193 Thymus body, 194, 230 Thyroglossal duct, 145, 227 Thyroid body, accessory, 227 development of, 194, 226

cartilage, 226

duct. 227

foramen, 404 Thyroids, lateral. 226. 228 Tissue fungus, 97 Toes, development of, 407 Tongue, development of, 143, 194 Tonsil, development of, 194 Tonsillar pit, 195 Trabecula; cranii, ;J87 Trachea, develoiuuent of, 225 Tragus, formation of, 358 Transverse colon, formation of, 203

crescentic groove, 80

fissure of brain. 2})H

processes of vertebne, 380 Trigeminal ganglion, 320 Trophoblast, 92

True chorion, 92

Truncus arteriosus, 113, 151, 154, 165 Trunk, skeleton of, development of^ 373 osseous stage of, 379 Trunk-muscles, development of, 363 Trunk -segments, 364 Tuber cinereum, 296, 300 Tubercles, corn icular, 226

cuneiform, 226 Tuberculum impar, 144, 194 Tubotympanic sulcus, 356 Tunica albuginea of ovary, 250 of testicle, 246 fibrosa, 30 propria, 30 vaginalis testis, 249 vasculosa, 29 lentis, 31^7 Turbinal folds. 361 Turbinate bone, inferior, ossification

of, 395 Turbinated bones, development of,

361 Twelfth pair cranial nerves, development of, 324 Twins, origin of, 59 Tympanic cavity, formation of, 194 membrane, 194

development of. 357 portion of temporal bone, development of, 393 Tympanohyal, 402

cartilage, 393 Tympanum, development of, 356

Umbilical aperture, 87, 186

arteries, 103. 165

cord, 102

hernia, congenital. 206

urinary fistula, 256

vein, 103.165, 169

vesicle, 80, 87, 186 function of, 89 human, 89

vessels, 103 Uncinate gyrus, 315 Unstriated muscle, development, 371 Urachus. 91, 256 Ureter. 237

development of, 232 Urethra, female. 257

male, formation of. 262

prostatic, formation of. 257 Urinary fistula, umbilical, 256 Urogenital aperture, 257

sinus, 190,196,256 Uskow, pillars of, 177 ,

Uterus bicomis, 2.53

development of. 253

double. 25.3

masculinus, 247. 257 Utricle, development of, 349 Uveal tract, development of, 340 Uvula, formation of, 137

432

INDEX,

Vagina, development of, 253

median septum iu, :i^ri Valve, coronary, IGl

Eustachian, 160

ofThebesius, 161

of V'ieussens, 294 Valves, atrioventricular, 156

auriculoventricular, 162

of heart, development of, 161

semilunar, development of, 163 Van Beneden's embryonic bud, 54 Vas aberrans, 247

deferens, formation of, 246 Vasa eflerentia, 246

recta, formation of, 246 Vascular area, 88

system, development of, 147, 409-416 fetal, final stage of, 182

tunic of eye, development of, 339 Vegetative pole, 27 Vein, <'ardinal, 164

hepatic, 181

iliac, leftc(mimon. development, 172

portal, development of, 181

renal, left, 173

superior mesenteric, 181

umbili(tal, 103 Veins, allantoic, 90, 164

cardinal, 164 anterior, 169 pdsterior, 1(>9

omphalomesenteric, 151

primitive ju^iular, 169

spermatic. 173

umbilical, 165, 169

vitelline, 151, 169 Velum interposi turn, 296. 297 Vena azygos major, 173 minor. 174

cava, inferior, 171. 174 superior, 170 Veiue hepaticie advehentes, 179

reveheiites. 179 Venous segmeut of heart, 156

system of fetus, 1()9 ])ort:il, 170 V'^iitral nu'Sfiitery. 190. 204 developuient of, 220

pancreas, 211 Ventricles, separation of, 158 Vermiform appendix. dcvelopment,203

process of cerebellum. 292 Virnix caseosa. 87, 121, 270 Vertfbni'. ossification of, 3^0 Vertebral bow, primitive, 376

column, <levelopment of. 373-3S2 meuibranous primordial. 375

])lati'. 65

region of primitive skull, 3'^7 Vesicle, blastodcrmii'. stage of, 49 tWi)-layered stage of, 52

germinal. 25, 27

lens-, 110

otic, 1(K», 34(5

umbilical, 80, 87, 18<)

Vesicles, cerebral, 287, 288 Vessel sprouts, 150 Vestibular ganglion, 351

nerve. 321 Vestibule of ear, development of, 352

of vagina, 259

of vulva, 257 Vestigial fold of Marshall, 172 Vieussens, valve of, 294 Villi of chorion. 93

of intestine, formation of, 206 Visceral arch, first, function of, 115, 131

arches, 112

metamorphosis of, 115 moqdiological significance of, 113

clefts, 112

layer of peritoneum, 189 of i)leum, 177

skeleton. 384 Vis«'eral-arch vessels, 113, 151. 165 Vitelline arteries, 151

artery, right, 152

circulation, formation of, 147

duct, 80, 87, 186

membrane, 25, 26

veins, 151, 169 Vitellus, 25, 26 Vitreous body, development of, 338

mesodermal, 338

primitive, 3,'i8 Voluntary' muscles, development, 363 Vomer, ossification of, 396

Weight of fetus at different stages, 412-416 at term. 122 Wharton, jelly of, 103 White commissures of cord, 285 tibrous tissue, formation of, 125 matter of brain, formation of. 30.3

of ct»rd. development of, 285 of hen's egg. 29

substance of Schwann, development of, 31 J». .|(»9. 41 Winslow. fonimen ol. 221 Wirsung, duct of. 212 Witches' milk. 276 WoltUan blastema, 236 body, 2:M duct. 235

in female. 253 ridge. 232. 234 Wolff's doctrine of epigenesis, 18 Wreath, 45

VoLK of ovum. 25 Yt»lk-sac, M), v87, 186

ZiNX, zcmule of, 3:58 Zona pellucida, 25. 31

r.idiata, 31 Zone, parirtal. 7(>

stem-, 76 Zonule of Zinn. .338 Zuckerkandl. organs of, 325

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Boston's Clinical Diag>nosis

Clinical Diagnosis. By L. Napoleon Boston. M.D., Associate in

Medicine and Director of the Clinical Laboratories. Medico-Chirurgical College, Philadelphia. Octavo of 563 page.'^. with 330 illustrations, many in colors. CInth, £4,00 net.

Dr. Boston here presents a practical manual of ihe clinical and laboratory examinations which furnish a guide to correct diagnosis, giving only such methods. however, which can be carried out by Ihe busy practitioner in his office as well as by the student in Ihe laboratory. In this new second edition the entire work has been carefully and thoroughly revised, Incorporating all the newest advances. Borton Modic&l Mid Sur^nl Journal

" Up h:is produccil a book winch muy he regarded emineolly as a praelical luid servioable guide. . . . The iliuslralions are both namerous and good."

■w^ T»« 

MATERIA MEDICA.

GET A • THE NEW

THE BEST /\m6riC2kn standard

Illustrated Dictionary

Just Issued— New (4th) Edition

The American Illustrated Medical Dictionary. A new and complete dictionary of the terms used in Medicine, Surgery, Dentistry^ Pharmacy, Chemistry, and kindred branches; with over lOO new and elaborate tables and many handsome illustrations. By W. A. Newman Borland, M. D., Editor of " The American Pocket Medical Dictionary," Large octavo, over 800 pages, bound in full flexible leather. Price, $^^0 net ; with thumb index, J5.00 net.

WITH aooo NEW TERMS

The immediate success of this work is due to the special features that distinguish it from other books of its kind. It gives a maximum of matter in a minimum space and at the lowest possible cost. Though it is practically unabridged^ yet by the use of thin bible paper and flexible morocco binding it is only i % inches thick. In this new edition the book has been thoroughly revised, and upward of two thousand new terms have been added.

Howard A. Kelly, M« D.» Professor of Gynecology, Johns Hopkins University, Baltimore.

"Dr. Dorland's dictionary is admirable. It is so well gotten up and of such convenient size. No errors have been found in my use of it."

Goepp's State Board Questions

state Board Questions and Answers. By R. Max Goepp, M. D.,

Professor of Clinical Medicine, Philadelphia Polyclinic. Octavo of

800 pages.

READY SOON

Every graduate who desires to practice medicine must pass a State Hoard Examination, and to aid him in successfully passing such an examination this work will be of inestimable value. Dr. Goepp has taken great pains to collect the many questions asked in the past by Boards of the various States, and has arranged and classified them under subjects in such a manner that the prospective applicant can acquire the knowledge on any branch with the least difficulty.

THE PRACTICE OF MEDICINE, ii

Hatcher and SoUmann's Materia Medica

A Text-Book of Materia Medica : including Laboratory Exercises in the Histologic and Chemic Examination of Drugs. By Robert A. Hatcher, Ph. G., M. D., of Cornell University Medical School, New York City ; and Torald Sollmann, M.D., of the Western Reserve University, Cleveland, Ohio. i2mo of 41 1 pages. Flex, leather, $2.00 net.

RECENTLY ISSUED— A NEW WORK

This work is a practical text-book, treating the subject by actual experimental demonstrations.

Journal of the American Medical

"The book is well written, the classifications are good, and the book is to be recommended as a practical guide in the laboratory study of materia medica."

Eichhorst's Practice

A Text-Book of the Practice of Medicine. By Dr. Hermann EiCHHORST, University of Zurich. Translated and edited by Augustus A. EsHNER, M. D., Professor of Clinical Medicine, Philadelphia Polyclinic. Two octavos of 600 pages each, with over 150 illustrations. Per set : Cloth, g6.cx) net ; Sheep or Half Morocco, $7.50 net

Bulletin of Johnt Hopkint Hospital

• • This book is an excellent one of its kind. Its completeness, yet brevity, the clinical

methods, the excellent paragraphs on treatment and watering-places, will make it very desirable."

Bridge on Tuberculosis

Tuberculosis. By Norman Bridge, A. M., M. D., Emeritus Professor of Medicine in Rush Medical College, in aflfiliation with the University of Chicago. i2mo of 302 pages, illustrated. Cloth.

Ji.50 net.

Medical News, New York

" Thoroughly representative of our practical methods of diagnosis and treatment of the disease."

12 SAUNDERS' BOOKS ON

Thornton's Dose-Book

Dose-Book and Manual of Prescription-Writinsr. By E. Q. Thornton, M. D., Assistant Professor of Materia Medica, Jefferson Medical College, Phila. Post-octavo, 392 pages, illustrated. Flexible Leather, $2.QO net.

Recently Issued— New (3d) Edition

Dr. Thornton, in making this revision, has brought his book in accord with the new (1905) Pharmacopeia. Throughout the entire work numerous references have been introduced to the newer curative sera, organic extracts, synthetic compounds, and vegetable drugs. To the Appendix, chapters upon Synonyms and Poisons and their antidotes have been added, thus increasing its value as a book of reference.

C. H. Kfiller. M. D.,

Professor of Pharmacology, Northwestern University Medical School, Chicago,

" I will be able to make considerable use of that part of its contents relating to the correct terminology as used in prescription-writing, and it will afford me much pleasure to recommend the book to my classes, who often fail to find this information in their other text-books."

Lusk on Nutrition

Elements of the Science of Nutrition. By Graham Llsk, Ph.D., Professor of Physiology in the University and Bellevuc Hospital Medical College. Octavo of 325 pages. Cloth, $2.50 net.

JUST READY

This practical work deals with the subject of nutrition from a scientific standpoint, and will be useful to the dietitian as well as the clinical physician. There are special chapters on the metabolism of diabetes and fever, and on purin metabolism.

Lewellyt F. Barlcer. M.D..

Professor of the Principles and Practice of Medicine^ Johns Hopkins University. " I shall recommend it highly. It is a comfort to have such ;i discussion of the subject."

Mathews' How to Succeed in Practice

How to Succeed in the Practice of Medicine. By Joskpii M. Mathews, M.D., LL.D., President Amerir^*^ ^edic»il Association, 1898-99. i2mo of 215 pages, illustrated. "CO net.

THE PRACTICE OF MEDICINE, 13

Gould and Pyle's Curiosities of Medicine

Anomalies and Curiosities of Medicine. By George M. Gould, M. D., and Walter L. Pyle, M. D. An encyclopedic collection of rare and extraordinary cases and of the most striking instances of abnormality in all branches of Medicine and Surgery, derived from an exhaustive research of medical literature from its origin to the present day, abstracted, classified, annotated, and indexed. Handsome octavo volume of 968 pages, 295 engra^rfngs, and 12 full-page plates.

Popular Editioa : Cloth, I3.OO net ; Sheep or Half Morocco, I4*00 net

As a complete and authoritative Book of Reference this work will be of value not only to members of the medical profession, but to all persons interested in general scientific, sociologic, and medicolegal topics ; in fact, the absence of any complete work upon the subject makes this volume one of the most important literary innovations of the day. •

The Lancet, London

"The book is a monument of untiring energy, keen discrimination, and erudition. . . . We heartily recommend it to the profession."

Saunders' Pocket Formulary

Recently Issued— New («th) Edition— Adapted to the New (ljK)5) Phannacopeia

Saunders' Pocket Medical Formulary. By William M. Powell, M. D., author of "Essentials of Diseases of Children"; Member of Philadelphia Pathological Society. Containing 1 83 1 formulas from the best-known authorities. With an Appendix containing Posological Table, Formulas and Doses for Hypodermic Medication, Poisons and their Antidotes, Diameters of the Female Pelvis and Fetal Head, Obstetrical Table, Diet-list, Materials and Drugs used in Antiseptic Surgery, Treatment of Asphyxia from Drowning, Surgical Remembrancer, Tables of Incompatibles, Eruptive Fevers, etc., etc. In flexible morocco, with side index, wallet, and flap. $i-7S riet

Johns Hopkins Hospital Bulletin

" Arran.£jed in such a way as to make consultation of it as easy as possible. It is remarkable how much information the author has succeeded in gettmg into so small a book."

TfD'SMS' Hd'Ol^S ON

SoUmann's Pharmacolo^

Including Therapeutics, Materia MedicB: Pharmacy, Prescription -writing. Toxicology, etc.

A Text-Book of Pharmacology. By Torald Sollmann, M.D.

Proretisor of Pharmacol. icry and Materia Medica, Medical Deiiartment of Western Reserve University, Clevjiand, Ohio, Handsome octavo vohjme of 1070 pages, fully illustrated. Cloth. S4.00 net.

RECENTLY ISSUCD—NEW [2d| EDITION

Because of the radical alterations which have been made in the new (1905) Pharmacupeia, it was found necessary to reset this book cnlirelv. The aulbi bases Ihe study of therapeutics on a systematic knowledge of Ihc nature and properties of drugs, and thus brings out forcibly the intimate relation betivee pharmacology and practical medicine.

J. r. Foflwrintflum, M. D.

/Vfl/. n/ Tkir.ifti,tici ,iaJ Tkrary airJ Praaic^o/ Prtrcriiiig Trinilt Med. CnUige. Tanid. " TJie work teriaiiiiy r>couplES groimd not covered in so conciu. luctul. and scientific mannor hv :iny oihi^i I<:>1 [ tiuvc re.id on Ihe ^ubjccla embm^ed."

Butler's Materia Medica

Therapeutics, and PharinacoIo{^

A Text-Book of Materia Medica, Therapeutics, and Pharmacologyk

By Georgf. F. Buti.er, Ph. G., M. D., Associate Profes.sor of Therapeutics. College of Physicians and Surgeons, Chicago. Revised bjr Smith Elv Jelliffe, M. D., Profes-ior of Pharmacognosy, Columb^ University. Octavo of 694 pages, iilustratcd. Cloth, S4.00 net; Half Morocco, 1S5.00 net.

RECENTLY ISSUED~NEW 15th EDITION Adapted to the New ( 1905) Pharmacopaa For this fifth edition Ur. Butlers textbook has been entirely remodeled, written, and re.sel. All obsolete matter has been eliminated, ami special at. lion has been given to the toxicologic and therapeutic effects of the newer c«  pniinds, A classification has been adopted which groups tOKelher those di the predominant action of which is on one system of organs.

M«dicnl Record. New York

■ Niillimg hiis littn omuled by the author whicli, J" hi. ■ijdgmtnl. «i pletenesa of lb« lexl, anrf Ihp itmlenl or general rej Ibe l,en.

bearini; upon Ibe value of ilruRs and remedi« lonr

PRACTICE, MATERIA MEDIC A, Etc. 15

The American Pocket Medical Dictionary. 4ihCd. Recently issued

The American Pocket Medical Dictionary. Edited by W. A. Newman Dorland, M. D., Assistant Obstetrician to the Hospital of the University of Pennsylvania. Containing the pronunciation and definition of the principal words used in medicine and kindred sciences, with 64 extensive tables. Flexible leather, with gold edges, ^l.oo net ; with thumb index, j$i.25 net.

"I can recommend it to our students without reserve."— J. H. Holland. M. D., of tkt Jefferson Medical College, PhilaeUlphia.

Vierordt's Medical Diagnosis. Fourth Editioii, Revised

Mkdical Diagnosis. By Dr Oswald Vierordt, Professor of Medicine, University of Heidelberg. Translated from the fifth enlarged German edition by Francis H. Stuart, A. M., M. D. Octavo, 603 pages, 104 wood cuts. Cloth, $4.00 net; Sheep or Half Morocco, %^.oo net.

• • Has been recoKnized as a practical work of the highest value. It may be considered indispensable

both to students and practitioners."— F. Minot, M. D., late Professor o/ Theory and Practice in Harvard University.

Cohen and Eshner's Diagnosis. Second Revised Editioii

EssENTiAi^ of Diagnosis. By S. Solis-Cohen, M. D., Senior Assistant Professor in Clinical M^icine, Jefferson Medical College, Phila. ; and A. A. Eshner, M. D., Professor of Clinical Medicine, Philadelphia Polyclinic. Post-octavo, 382 pages; 55 illustrations. Cloth, $1.00 net. /// Saund^rs^ Question -ComJ^^nd Series.

"Concise in the treatment of subject, terse in expression of fact." — American Journal of the Medical Sciences.

Recently Issued

Morris' Materia Medica and Therapeutics. New (7th) Edition

ESSENTIAI^S OF MATERIA MeDICA, ThERAPEITICS, AND pRKSCKinioN-WRn ING.

By Henry Morris, M. D., late Demonstrator of Therapeutics, Jefferson Medical College, Phila. Revised by AV. A. Bastedo, M. D., Instructor in Materia Medica and Pharmacology iit Columbia Univer.sity. 1 2mo, 300 pages. Cloth, ^ 1. 00 net. In Saunders* Question- Compcnd Series.

• • Cannot fail to impress the mind and instinct in a lasting manner." — Buffalo Medical Journal.

Williams' Practice of Medicine Recently issued

• Essentials of tiik Practick ok MiniriNi. By W. R. Williams, M.D.. formerly Instructor in Medicine and Lecturer on Hygiene, Cornell Universiiv ; and Tutor in Therapeutics, ('<.lumbia University, X. V. l2mo of 450 pnge«-, illustrated. In Satmdrrs* QnestioH-ConiptnJ Series. Double nuniler, $1.75 net.

Stoney's Materia Medica for Nurses N^S^ i£n

Materia Medica ior Nirsivs. By Emily M. A. Stoney. Supvrintmdent of the Training School for Nurses ;it the Cnrney Hr>spital. South Boston, Mass. Handsome i2mo vohime of 300 pai^es. Cloth. 51.50 net.

"It contains about everything that a nurse ouKht to know in reg^ard to Atvl%^."— ^Journal of the American Medical Association.

Grafstrom's Mechano-therapy Seco^d*E&n!^ia,««l

A Text- Book of Mf( hano-therapy (Massai^e and .Medical GymnasticsL Bv Axel V. Grafstrom. B. Sc, M. D.. .\ttendini,' Physician to Augustus Adolphus Orphanage, Jamestown. N. Y. i2mo, 200 piijes, illustrattMl, 51-25 net.

"Certainly fulfills its mission in rcnderinR^ comprehensible the subjects of massage and medical gymnastics." — Xew Vork Medical Journal.

i6 SAUNDERS' BOOKS ON PRACTICE, Etc.

Jakob and Eshner's Internal Medicine and Diagnosis

Atlas and Epitome op Internal Medicine and Clinical Diagnosis. By Dr. Chr. Jakub, of Erlangen. Edited, with additions, by A. A. Eshner, M. D., Professor of Qinical Medicine, Philadelphia Polyclinic. With 182 colored figures on 68 plates, 64 text- illustrations, 259 pages of text. Cloth, j^j.oo net. In Sounder^ Hand- Atlas Series.

• • Can be recommended unhesitatingly to the practicing physician no less than to the student." —

Bulletin 0/ J^knx Hopkins Hospital.

Lockwood's Practice of Medicine. Re^^ia^ed

A Manual of the Practice of Medicine. By Geo. Roe Lockwood, M. D., Attending Physician to the Bellevue Hospital, New York City. Octavo, 847 pages, with 79 illustrations in the text and 22 full-page plates. Cloth, ^4.00 net.

A work of positive merit, and one which we gladly welcome."— AV«v York Medicml Joumai.

Keating's Life Insurance

How TO Examine for Life Insurance. By the late John M. Keating, M. D., Ex-President of the Association of Life Insurance Medical Directors. Royal octaro, 211 pages. With numerous illustrations. Cloth, j$2.oo net. *

" This is by far the most useful book which has yet appeared on insurance examiaation." — Medicml New*.

Corvrin's Physical Diagnosis. Thiid Cditioii. Revised

Essentials of Physical Diagnosis of the Thorax. By A. M. Corwin, A. M., M. D., Professor of Physical Diagnosis, College of Physicians and Surgeons, Chicago. 220 pages, illustrated. Cloth, flexible covers, j$i.25 net.

" A most excellent little work. It arranges orderly and in sequence the various objective pheaomena to logical solution of a careful diagnosis."— y<7vr»a/ 0/ Nervous and Mental Diseases.

Barton and Wells* Medical Thesatnrus

A Thi<:saurus of Medical Words and Phrases. Hy W. M. Barton, M. D., and W. A. Weli-S, M. D., of Georgetown University, Washinjjion, D. C. l2mo of 535 pages. Flexible leather, $2.50 net ; thumb indexed, $3.00 net.

Jelliffe's Pharmacognosy Recently issued

An Introduction to Pharmacognosy. By Smith Ely Jelliffe, Ph. D,, M. D., of Columbia University. Octavo, illustrated. Clolh, I2.50 net.

Stevens' Practice of Medicine. New (7th) Edition— Recently issued

A Manual of thk Practice or Medicinf. By A. A. Stevens. A. M., M. D.,

Professor of Pathology, Woman's Medical College, Phila. Specially intended for

students preparinp for j^raduaiion and hospital examinations. Post-octavo, 556 pages; illustrated. Flexible leather, $2.50 net.

"An excellent condensation of the essentials of medical practice f«ir the .student, and may be found also an excellent reminder for the bu»y physician." — Buffalo Mt'dical Journal.

Paul's Materia Medica for Nurses just Ready

Materia Mkhica for Nursf:s. By Georc.e P. Paui, M.D., Assistant Visiting Physician and Adjunct Radiographer to the Samaritan Hospital, Troy, N. Y. i2mo of 240 pages. Cloth, ^1.50 net.

In Dr. Paul's new work the physiologic actions of the drui^s an: arranged according to the action of the drug and not the organ acted upon. Another important section is that on pretoxic signs, giving the warnings of the full action or the bejjinninij toxic effects of the drug. If these signs be known many cases of drug poisoning may be prevented.